Would / going to

I would like to know what is the difference between would and is going to, for instance:

How long would it take? and How long is going to take?

Another example:

It would effect you and It is going to effect you

Many thanks

Hello Naser and thanks very much for your question!  Now, first of all, before we answer your question, I'd like to have a quick look at the grammar of what you wrote. We need to add a subject to your sentence. You said How long is going to take? and in fact we need a subject in there so it would say How long is it going to take? And regarding your use of the word effect, you need to use it with an a not an e.

Right, now we've sorted your questions out, let's have a look at the difference between the two forms, that's would and going to. And here we've got an example conversation:

Woman:	I've just heard - my PhD proposal's been accepted! I'm going to start my research next year.
Man:	How exciting! How long is it going to take?

The woman has used going to because she's talking about future plans, that's things which are  definitely going to happen. So the man also uses going to because he's asking for more information about the plans.

Now, we can also use going to, not just for certainties but for things which are probably going to  happen. So, sometimes we use going to with 1st conditional sentences to express a strong possibility or probability. Here's an example:

If you drink the whole bottle of medicine, it is going to affect you very badly.

And that was a 1st conditional sentence with if and present simple with going to, which shows the likely result.

We can also use modal verbs and first conditionals, like this:

If you drink the whole bottle of medicine, it might affect you very badly.

So the modal verb with that 1st conditional sentence is might, and it shows quite a strong possibility.

Now let's have a look at the modal verb would. And here's an example dialogue:

Woman:	Are you thinking of doing a masters' degree?
Man:	Oh, they're so expensive! I can't afford to stop work, so I'd have to do it part-time.
Woman:	Part-time? How long would it take?

Now this time, we're not talking about plans, and we're not talking about strong possibilities or something that's probably going to happen. With this sentence, we're talking about possibilities, things we're thinking about, but we haven't made a plan about them. So, in this situation, we use modal verbs like would and could to express the idea of smaller possibility, or improbability, or even impossibility, like we do with 2nd conditionals. Here's an example of a 2nd conditional with would to express a small possibility:

If you drank the whole bottle of medicine, it would affect you very badly.

The difference, then, between would and going to: going to we often use for plans, or things that we think are probably going to happen, like we do with 1st conditional sentences. But for things which are less likely to happen, impossible, improbable or even just talking about dreams and ideas, we use 2nd conditional sentences and we use would.

Thanks for your question Naser, and I do hope my answer's going to be useful!