Aome more clarity on が; when used with a verb, the verb must be intransitive, unless the subject is performing an action (transitive) on another object.
The use of transitive and intransitive verbs aren't normally taught to beginners, but I think it's an important part of learning particles. Here's a short list. even numbers are transitive, and odd numbers are their intransitive counterparts.
1. 開ける to open (as in "I open the door")
2. 開く to open (as in "the door is open")
3. 閉める to close ("I close the door")
4. 閉まる to close ("the door closes")
5. 伸ばす to grow ("John's growing a beard")
6. 伸びる to grow ("your hair has grown a lot!")
7. 沸かす to boil ("go boil some water")
8. 沸く to boil ("the water is boiling already)
and so on...
These are some examples of special verbs that have transitivity pairs; there is a way to conjugate most verbs to make them intransitive, but I'm getting ahead of myself here....
Here are some sample phrases using transitive versus intransitive verbs:
ドアが開いている。
the door is open
マイクさんがドアを開ける。
mike opens/will open the door
Notice that in the first sentence where I use the intransitive verb, and the door is the subject, whereas the second sentence uses the transitive verb, and the door becomes the object receiving the action from the subject. This is a key distinction to make when using particles with these kinds of verbs.
you could never use the transitive 開ける with ドア, unless you have a subject performing the action, so ドアが開ける is right out. The door itself is opening, it is not opening something else.
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another way to look at が is to think of it as a marker for new information, which is why it is used alongside the question words, the "do" part of the "こそあど" set (dore, dare, docchi, donna, doko). There are of course other uses for it, but i found this useful when deciding when to use it in the beginning. Not only is が used with the question words, but it is also used with the answer. This is because the question is asking for new information, and in the answer you are providing the conversation a new subject.
so when someone asks
誰がドアを閉めた?
who closed the door?
The answer would be
マイクさんがドアを閉めた。
Mike closed the door.
I can not think of a situation where a question word would be followed by は... so that's certainly something to avoid.
Particles are very challenging, I hope some of this helps. 頑張って下さい。