The second and third conditionals are both types of conditional sentences in English, and they are used to talk about hypothetical or unreal situations. Here’s a breakdown of the differences between them:

1. Second Conditional (Present Unreal Conditional):

• Form: If + past simple, would + base verb 
• Example: If I had a million dollars, I would buy a mansion.
• Usage: The second conditional is used to talk about unreal or unlikely situations in the present or future. It suggests a hypothetical condition and its probable result. However, the condition is unlikely or impossible to happen.

In the example, the speaker doesn’t have a million dollars (unreal condition), and the result is the hypothetical action of buying a mansion.

2. Third Conditional (Past Unreal Conditional):

• Form: If + past perfect, would have + past participle
• Example: If I had studied harder, I would have passed the exam.
• Usage: The third conditional is used to talk about unreal or hypothetical situations in the past. It describes a situation that did not happen because the condition was not fulfilled. In the example, the speaker did not study hard enough (unreal condition), and as a result, they did not pass the exam. The regret or consequence is in the past.

So, the main difference lies in the time frame of the unreal situation. The second conditional deals with present or future unreal conditions and their probable outcomes, while the third conditional deals with past unreal conditions and the hypothetical outcomes that would have occurred had the condition been fulfilled.

Here you can read more about The Second Conditional.
Here you can read more about The Third Conditional.

Similarities between Second and Third Conditionals

While the second and third conditionals have key differences, they also share some similarities in terms of structure and general usage:

1. Both are Conditional Sentences:

• Second and third conditionals are both types of conditional sentences used to express hypothetical or unreal situations.

2. Similar Structure:

• Both conditionals involve an “if” clause and a main clause.
• Second Conditional: If + past simple, would + base verb
• Third Conditional: If + past perfect, would have + past participle

3. Unreal or Hypothetical Situations:

• Both conditionals deal with situations that are not real or did not happen exactly as described.
• Second Conditional: Unreal conditions in the present or future.
• Third Conditional: Unreal conditions in the past.

4. Use of “Would”:

• Both conditionals use “would” to express the hypothetical or unreal outcome.
• Second Conditional: would + base verb
• Third Conditional: would have + past participle

5. Implies a Consequence:

• In both conditionals, there is a consequence or result based on the hypothetical condition not being fulfilled.
• Second Conditional: The consequence is in the present or future.
• Third Conditional: The consequence is in the past.

Despite these similarities, it’s important to note that the primary distinction between the second and third conditionals lies in the timeframe of the hypothetical situations—the second conditional deals with present or future unreal conditions, while the third conditional deals with past unreal conditions.

Common Mistakes and How to Avoid Them second and third Conditionals

Choosing between the second and third conditionals can be tricky for English learners. Here are some common mistakes and tips on how to avoid them when deciding which conditional to use.

Common Mistakes:

1. Using the wrong conditional for the time frame:

• Mistake: “If I would have dinner with him last night, I would tell him the news.”
• Correction: “If I had had dinner with him last night, I would have told him the news.”
• Tip: The second conditional is for unreal conditions in the present or future, while the third conditional is for unreal conditions in the past.

2. Not using the past perfect tense in the third conditional:

• Mistake: “If I studied harder, I would have passed the exam.”
• Correction: “If I had studied harder, I would have passed the exam.”
• Tip: The “if” clause in the third conditional should use the past perfect tense.

3. Confusing the order of the clauses:

• Mistake: “I would have passed the exam if I would have studied harder.”
• Correction: “I would have passed the exam if I had studied harder.”
• Tip: Maintain the correct word order with the “if” clause first in the sentence.

4. Using “will” or “going to” in the result clause:

• Mistake: “If she had called me, I will help her.”
• Correction: “If she had called me, I would have helped her.”
• Tip: In both conditionals, use “would” in the result clause, not “will” or “going to.”

5. Overusing “would” or modal verbs:

• Mistake: “If he would come to the party, I would be happy, and we would have fun.”
• Correction: “If he came to the party, I would be happy, and we would have fun.”
• Tip: Keep your sentences concise and avoid unnecessary repetition of modal verbs like “would.”

Practical Applications and Usage of second and third Conditionals

The second conditional often referred to as the “present unreal” conditional, is used to express hypothetical situations in the present or future along with their imagined outcomes. It is commonly utilized to give advice, make recommendations, or express desires that are unlikely to happen. An example of its practical application would be if someone were to say, “If I were rich, I would travel the world,” where the condition of being rich is unlikely but the desire to travel is present.

On the other hand, the third conditional, also known as the “past unreal” conditional, is used to talk about hypothetical situations that have already taken place in the past. It is frequently employed to express regret or describe situations that did not actually happen. An instance of its usage would be if someone were to say, “If I had studied harder, I would have passed the exam,” where the regret of not studying harder is combined with the imagined outcome of passing the exam.

Examples of Second and Third Conditionals with Explanation of Difference

1. Second Conditional: “If I won the lottery, I would buy a new house.”Third Conditional: “If I had won the lottery, I would have bought a new house.”Explanation: The second conditional talks about a hypothetical situation in the present or future (winning the lottery) and its unreal result (buying a house). The third conditional discusses a hypothetical past situation (not winning the lottery) and its past result (not buying a house).

2. Second Conditional: “If I were a bird, I would fly around the world.”Third Conditional: “If I had been a bird, I would have flown around the world.”Explanation: The second conditional imagines a present unreal scenario (being a bird now) and its potential outcome (flying around the world). The third conditional reflects on an impossible past scenario (being a bird in the past) and its unrealized outcome.

3. Second Conditional: “If she spoke French, she would move to Paris.”Third Conditional: “If she had spoken French, she would have moved to Paris.”Explanation: The second conditional presents a current hypothetical ability (speaking French) and a potential decision (moving to Paris). The third conditional looks at a past unreal situation (not speaking French) and its consequent decision that didn’t happen.

4. Second Conditional: “If we were friends, I would help you.”Third Conditional: “If we had been friends, I would have helped you.”Explanation: The second conditional suggests a present hypothetical relationship (being friends now) and its possible effect (helping). The third conditional reflects on a past hypothetical relationship (not being friends in the past) and its missed opportunity for help.

5. Second Conditional: “If it rained tomorrow, we would cancel the picnic.”Third Conditional: “If it had rained yesterday, we would have canceled the picnic.”Explanation: The second conditional considers a future hypothetical event (rain tomorrow) and its potential consequence (canceling the picnic). The third conditional discusses a past hypothetical event (rain yesterday) and a decision that would have been made.

6. Second Conditional: “If he knew the truth, he would be upset.”Third Conditional: “If he had known the truth, he would have been upset.”Explanation: The second conditional implies a present unreal condition (knowing the truth now) and its hypothetical reaction (being upset). The third conditional talks about a past unreal condition (not knowing the truth then) and the reaction that didn’t occur.

7. Second Conditional: “If she were available, we would invite her to the meeting.”Third Conditional: “If she had been available, we would have invited her to the meeting.”Explanation: The second conditional indicates a present hypothetical (being available now) and its potential action (inviting to the meeting). The third conditional reflects on a past situation (not being available) and the action that wasn’t taken.

8. Second Conditional: “If I had enough money, I would start my own business.”Third Conditional: “If I had had enough money, I would have started my own business.”Explanation: The second conditional talks about a present hypothetical financial situation (having enough money now) and a future aspiration (starting a business). The third conditional refers to a past financial situation that wasn’t true (not having enough money) and a missed opportunity.

9. Second Conditional: “If they were more experienced, they would take on the project.”Third Conditional: “If they had been more experienced, they would have taken on the project.”Explanation: The second conditional suggests a current hypothetical condition (being experienced now) and a possible decision (taking on the project). The third conditional looks back at a past condition (lack of experience) and a decision that was not made.

10. Second Conditional: “If I ate less junk food, I would be healthier.”Third Conditional: “If I had eaten less junk food, I would have been healthier.”Explanation: The second conditional proposes a present hypothetical lifestyle change (eating less junk food now) and its beneficial outcome (being healthier). The third conditional reflects on a past lifestyle (eating junk food) and its impact on health that cannot be changed now.