To talk about hypothetical situations we use the past form of an action after if, then 'would' and the basic form of another action:
(We can use 'were' instead of 'was' with 'I' and 'he/she/it'. This is mostly done in formal writing).
It has two uses.
First, we can use it to talk about things in the future that are probably not going to be true. Maybe I'm imagining some dream for example.
Second, we can use it to talk about something in the present which is impossible, because it's not true. Is that clear? Have a look at the examples:
http://www.perfect-english-grammar.com/second-conditional.html (Perfect English Grammar)