Conditional present or past

Which one is correct?

1. What would happen if they stop paying you?


2. What would happen if they stopped paying you?

In what context we can use this? after an event happened or before that.

Answer

The phrase "what would happen if" indicates that we are talking about something that has not happened yet and is unlikely to happen. We are discussing a hypothetical situation, and the consequences of that situation.

When we make a sentence about a hypothetical situation, we indicate that it is hypothetical by backshifting the tense of the situation- so present becomes past simple, past simple becomes past perfect, etc. We use the same trick for reported speech.

In principle, the second example is the correct answer, because present simple stop is backshifted to past simple stopped. If you are writing something, this is definitely the right option to use.

In everyday speech, you will hear people describing hypothetical situations using present simple. Maybe the logic is that they are talking about a future event will stop, which gets backshifted to present simple stop.

How do we know whether a situation is hypothetical or not?

If you don't stop, I will kill you

This is a simple conditional- if this (possible) situation happens, the consequence will be this... no hypothesis. Tenses are present and future.

If I were rich, I would buy a heliocopter"

This is understood to be pure hypothesis as I am not rich, so we use the subjunctive.

It's not clear cut, but we tend to use the subjunctive (expressed in most cases as a backshift) in order to give the impression that we think something is unlikely. In the OP's question, the subjunctive is used to talk about a situation that we hope is unlikely.

Here is some more information about the different types of conditionals, but remember that these are simplified guidelines for people learning the language. Actual usage may vary.