Second conditional

if + past subjunctive | would + infinitive (simple or continuous)

The second conditional can describe an unreal situation with reference to the present. We imagine a hypothetical situation which contrasts with reality:

If I knew the answer, I'd tell you.

Here If I knew the answer means that I don't know the answer.

In the if-clause, we use the past subjunctive form of the verb, which is identical to the past simple tense except in the case of the verb be. Traditionally, the past subjunctive form of be is were for all persons including the first and third person singular. However, nowadays I/he/she/it was is more common, while were is mainly used in formal styles and in the set phrase if I were you:

If I lived by the sea, I would be lying on the beach right now.
If he was/were taller, he would make a good basketball player.
If I were you, I'd stop smoking.

The second conditional can also express a theoretically possible but unlikely future situation. In the following examples, the conditions in the if-clause are possible but are not likely to be fulfilled:

If the Third World War broke out, it would probably mean the end of humankind.
If you missed the 6 o'clock train, you wouldn't get here before 7.
If I lost my job, I would be in serious trouble.

Compare the first and second conditionals when they refer to possible future actions:

If you miss the 6 o'clock train, you won't get here before 7. (first conditional, the condition is probable, you may miss the train)

If you missed the 6 o'clock train, you wouldn't get here before 7. (second conditional, the condition is possible but unlikely, you probably won't miss the train)

Sometimes a sentence may look like a second conditional, but in fact it is not:

If he was angry, he would always shout and slam doors.

This is actually a zero conditional that refers to past time. Here was in the if-clause is not a past subjunctive but the past tense of be, while would in the main clause expresses repeated past actions.