Quick answer
Past tense + бы = would. Я хоте́л бы = I would like. Е́сли бы я знал, я бы сказа́л = if I had known, I would have said.
How it is built
There is no separate conditional tense and no difference between “would” and “would have” — the context does that work.
е́сли бы — if
Both halves take бы, and both verbs are in the past. For a real, possible condition you use е́сли without бы: Е́сли бу́дет вре́мя, приду́.
Where you will use it first
Я хоте́л бы — the polite “I would like”, in every shop and restaurant. That sentence alone justifies learning the whole structure.
Typical mistakes
Я хочу́ бы
Я хоте́л бы
бы attaches to the past tense, never the present.
Е́сли бы бу́дет вре́мя
Е́сли бу́дет вре́мя / Е́сли бы бы́ло вре́мя
Either a real condition with the future, or an unreal one with бы and the past.
Check yourself
How do you say “I would like a coffee” (a woman speaking)?