Comparison with a hypothetical

Question

However, if people used their own cars, the amount of harmful gases would be much more than it would if they depend on public transformation system

I am not good at using would and than. Is the sentence above correct?

Answer 1

However, if people used their own cars, the amount of harmful gases would be much more than it would if they depend on public transformation system.

I am not good at using would and than. Is the sentence above correct?

Many native English speakers are not good at it either! Following the rules for expressing a supposition is surprisingly difficult.

Your sentence is understandable but we can improve it in several ways.

First, as GreaseMonkey pointed out, "greater than" is better than "more than" in this case. This is a subtle point that many native English speakers get wrong. The general rule is to use "more than" or "fewer than" when the thing is divided up into discrete units. If it were apples or children or dogs, you would use "more than". Gas isn't divided up into discrete units, so use "greater than" or "less than".

However, if people used their own cars, the amount of harmful gases would be much greater than it would if they depend on public transformation system.

Second, you said "transformation" where you meant "transportation". Transformation is turning one object into another: the vampire transforms into a bat.

However, if people used their own cars, the amount of harmful gases would be much greater than it would if they depend on public transportation system.

Third, "public transportation system" needs an article, either "a" or "the". Oddly enough "public transport" does not get an article and is idiomatic, so I'd go with that.

However, if people used their own cars, the amount of harmful gases would be much greater than it would if they depend on public transport.

Fourth -- to get to the thrust of your question -- you would typically modify the verb with "had" to indicate that a supposition is being made, and you would make sure that "used" agreed in tense with "depend".

I might have said:

However, if people had used their own cars, the amount of harmful gases would have been much greater than it would have if they had depended on public transport.

And you can actually eliminate the "if"s like this:

However, had people used their own cars, the amount of harmful gases would have been much greater than it would have had they depended on public transport.

Now a non-native English speaker would be justified in pointing out that this is thoroughly ridiculous. How many auxiliary verbs are we going to cram into that sentence? "Had he wanted to have arrived on time he would have had to have driven" is grammatical (I hope!) but the number of times the verb "have" appears is crazy.

If you wanted to make that less wordy this would be acceptable:

However, had people used their own cars, the amount of harmful gases would have been much greater than had they depended on public transport.

The "it would have" is implied.

Fifth, there's a missing verb.

However, had people used their own cars, the amount of harmful gases produced would have been much greater than had they depended on public transport.

And sixth, you are using "depended on" colloquially here and that's fine; the sentence makes sense. But if this were being closely analyzed one might think that you were trying to make a distinction between people who use their cars and people who have no other choice but to depend on the bus. It would be perfectly acceptable to use "used" twice:

However, had people used their own cars, the amount of harmful gases produced would have been much greater than had they used public transport.