Unlike noun sentences or adjective sentences, が is more commonly used in verb sentences. が indicates the subject, and the sentence describes a movement, action or phenomenon. This kind of sentence is a neutral description sentence, and が used in these sentences is called 中立叙述の"が" (が of neutral description). Actually, the topic is one of the information in this neutral description sentence, where the focus of the next sentence is put on to continue the conversation.
e.g.
昔々、あるところに、おじいさんとおばあさんが住んでいました。(a neutral description sentence)
おじいさんは山へ柴刈りに行きました。 (おじいさん is the topic quoted from the previous sentence. This topic sentence describes the action of the topic here.)
If が is used for already-known information instead of は, が emphasizes the exclusive nuance. For instance, if おじいさんが山へ柴刈りに行きました。 follows the first sentence above, it emphasizes "it is the old man who went to the mountains to gather firewood, not the old woman or someone else." This が is called 指定の"が" (が for specifying). Actually, が in noun sentences is this 指定の"が".
Incidentally, 私, あなた or これ/それ/あれ are treated as already-known information even if it's not mentioned previously. Thus, は is the most common for these words even in verb sentences, and が works as the specifying one. See the following threads in detail.
は, が and を what's the difference between the following sentences? | Japan Forum
STILL confused about は and が; example sentences | Japan Forum
As you can see, verb sentences are more complicated than adjective sentences or noun sentences. The function of は/が can be different depending on the context, therefore the meaning is often ambiguous just by a stand alone sentence. In other words, ドアが閉まります and ドアは閉まります can have different nuance depending on the situation where they are used. (neutral description, general statement, a near future event, an explanation about already-known information, emphasizing "the door", contrasting "the door", etc., etc.)
Notice that these explanations are just a summary. Textbooks teach these things gradually in proper way and appropriate order. That's why we repeatedly recommend using decent textbooks as a learning material. I don't think you can fully understand these essential grammatical issues just by translating Japanese song lyrics or from answers to the questions you come across in your mind randomly in the forums like here, as I pointed out previously.