Many times i get confused whether i should use 's with a noun or not. For example:
I want to know why and when we use one or the other. What do you call such a group of words? Is it a phrase?
It should be noted that English is not my native language so pardon my ignorance.
The 's signifies possession. You add it when you want to stress that the second noun is the property of the first one. Without the 's, it's a more generic version.
For example, if you bring a friend to your garage, and he sees a bunch of bolts and screws lying on a workbench and asks "What're those?" then you can say "They are engine parts", and you would mean that they are things that go into engines, but they don't belong to any engine specifically, or it doesn't matter which engine they belong to. They might just be some parts you have for a later repair.
On the other hand, you could also say "Those are the engine's parts", which would indicate that they came from a specific engine and they still belong to it. Probably because you're repairing an engine and you removed them from it for cleaning, and they will be put back in later.
So whether to use the 's or not is usually part of the context; do you want to say that "the parts" belong to "the engine" or do you want to say that "the parts" are for "an engine"?
(same with the others, although they are less likely to come up.)
It's context-dependent.
Consider the following:
The first two both work, while the "engine's" version of #3 requires a determiner, producing something like "This engine's parts are getting very expensive." However, doing this changes the sense of the sentence from engine parts in general to the parts of one specific engine.
