You're imagining something that isn't real yet. That's the condition that calls for would, normally in the result clause of a conditional. Match your exact situation to the examples below, from requests to past routines.
Don't put would in the if-clause. Write If I had time, I would help you, not If I would have time. The if-clause states the condition, so it uses the past simple, and would belongs in the result clause, where it shows what would happen if that condition were true. One set courtesy phrase keeps it: If you would be so kind as to sign here.
Would in real, imagined, and reported situations you'll actually write
You're picturing something that isn't real yet. If you're describing an imaginary result, put would in the result clause: If I had more time, I would learn another language. The if-clause sets the condition and normally doesn't contain would.
You're asking someone for a favor and want to sound considerate. Turn the request into a would question: Would you open the window? It softens the directness of Will you open the window?
You're recalling a repeated past routine, not a single event. With a past time frame and a frequency phrase, use would plus the base verb: Every summer, we would drive to the coast. One-time events take the simple past instead.
You're describing a past state rather than a repeated action. Would doesn't fit past states like live, be, have, or know; say We used to live in Leeds, not We would live in Leeds. It's wrong there, not just weaker.
You're reporting what someone said, and your reporting verb is in the past. Will commonly backshifts to would: She said she would call me later. If her promise is still clearly true or relevant now, you can keep will: She said she will call me later.
You're weighing would against will for a request. Choose will when you mean a straightforward future or a plain instruction; choose would when you want the same request to sound gentler: Will you help me? versus Would you help me?
Keep would out of the if-clause
In a standard conditional sentence, would stays in the result clause and stays out of the if-clause. Say If I had time, I would help you, not If I would have time, I would help you. The only exception is the fixed formal courtesy phrase, such as If you would be so kind, which you use as a whole expression rather than building freely.
You're writing a conditional sentence and you want to place would somewhere. Put it in the main clause, where the imagined result lives: If we left now, we would catch the train. The if-clause just states the condition, so If we would leave now reads as an error.
One polite exception exists, and it's fixed in form rather than a free pattern. In formal requests like If you would be so kind as to sign here, would does appear in the if-clause. Treat it as a set courtesy phrase, not a model for ordinary conditionals.
In speech, you'll sometimes hear If I would have known from native speakers, so don't be thrown by it. It's still considered wrong in formal writing and exams, so keep the standard pattern there: If I had known, I would have called.
You're advising someone about an unlikely present or future choice, like whether to apologize. Use the pattern where the if-clause takes the past form and the main clause takes would plus the base verb: If I were you, I would apologize.
The were form in that if-clause matters mainly in formal writing and exams. If I was you is increasingly common in speech and rarely corrected there, but stick with were when your sentence will be graded or printed.