"Second conditional" or "conditional II" refers to a pattern used to describe hypothetical, typically counterfactual situations with a present or future time frame (for past time frames the third conditional is used). In the normal form of the second conditional, the condition clause is in the past tense (although it does not have past meaning), and the consequence is expressed using the conditional construction with the auxiliary would:
If I liked parties, I would attend more of them.
If it rained tomorrow, people would dance in the street.
Here are some more examples:
If I won the lottery, I would buy a big house.(I probably won't win the lottery)
If I met the Queen of England, I would say hello.
She would travel all over the world if she were rich.
She would pass the exam if she ever studied.(She never studies, so this won't happen)
The past tense (simple past or past progressive) of the condition clause is historically the past subjunctive. In modern English this is identical to the past indicative, except in the first and third persons singular of the verb be, where the indicative is was and the subjunctive were; in this case either form may be used. (Was is more colloquial, and were more formal, although the phrase if I were you is common in colloquial language. For more details see English subjunctive: Use of the past subjunctive.)
If I (he, she, it) was/were rich, there would be plenty of money available for this project.
If I (he, she, it) was/were speaking, you would not be allowed to interrupt like that.
When were is the verb of the condition clause, it can be used to make an inverted condition clause without a conjunction. If the condition clause uses the past tense of another verb, it may be replaced by the auxiliary construction was/were to + infinitive (particularly if it has hypothetical future reference); if this is done and were is used, then inversion can be applied here too:
If I was rich, ... / If I were rich, ... / Were I rich, ...
If I flew, ... / If I was/were to fly, ... / Were I to fly, ...
Another possible pattern is if it wasn't/weren't for... (inverted form: were it not for ...), which means something like "in the absence of ...".
The conditional construction of the main clause is usually the simple conditional; sometimes the conditional progressive (e.g. would be waiting) is used. Occasionally, with a first person subject, the auxiliary would is replaced by should (similarly to the way will is replaced by shall). Also, would may be replaced by another appropriate modal: could, should, might.
When referring to hypothetical future circumstance, there may be little difference in meaning between the first and second conditional (factual vs. counterfactual, realis vs. irrealis). The following two sentences have similar meaning, although the second (with the second conditional) implies less likelihood that the condition will be fulfilled:
If you leave now, you will still catch your train.
If you left now, you would still catch your train.
Notice that in indirect speech reported in the past tense, the first conditional naturally changes to the second: