It’s a little bit embarassing but it’s only come to light more recently as I started trying to write.
I’m not really sure when to use です or がある・がいる.
I understand that when I want to indicate possession; I have, then sure it’s the the later.
But both grammar points have ‘to be’ in their titles.
です replaces だ (the copula) in polite speech, although only at the end of sentences or in direct quotes (where you’re quoting that someone said です). Both ある and いる mean “to be,” but the former is only used for inanimate objects and the latter for animate. Both also have additonal senses. For instance, you already know ある can also mean “to have.” いる can also have this meaning, but in an existential sense. For example, 僕には弟がいます (I have a younger brother–lit. to me there exists a younger brother).
です means “is/am/are”
ある・いる means “there exists” or “there is”
AはBです means “A is B”. You can’t replace it with “there exists”.
田中さんは先生です。“Tanaka is a teacher.” It doesn’t make sense to say “Tanaka there exists a teacher.”
Aがある means “There exists A”. This is also translated as “I have A” depending on the situation.
私は弟がいます。“As for me, there exists a brother younger than me.” or “I have a younger brother.” It doesn’t make sense to say “I am a brother younger than me”.
(I realized as I was typing this that it I couldn’t use “younger brother” to make my point, because “I am a younger brother” still makes sense in English, but 弟 implies someone else younger than you.)
ぺんです。It is a pen.
ぺんがある。Literally, A pen exists. => There is a pen
ペンです。It is a pen.
Really, this is missing a hidden subject, so it’s really “[It] is a pen”.
[それは] ペンです。[それが] ペンです。[It/That] is a pen. それ is a hidden subject.
(Notice I’m calling it “subject”, which as you know could be marked with subject が or topic は, depending on the situation.)
You can replace です with an equal sign.
it = pen
For context, this is the answer to a question like this:
このへんなものは、何ですか！What is this weird thing!
それはペンです。That is a pen.
ペンがある。Literally, A pen exists. => There is a pen
Here, you’re talking about the existence of a pen, not that it’s equal to something.
The pen is the subject. You’re describing that the pen exists in the world, not describing some characteristic of it.
For context, this is the answer to a question like this:
ない！ない！いいアイデアを書きたいんですが… There’s nothing! There’s nothing! I want to write a great idea, but…
ほら！あそこに！ペンがある！Hey! Over there! There’s a pen!
a little spot of understand might be forming in my brain… I think I would need to come back with some sentences when I try to write them… mull it over a bit
thanks for explanation
です can also be used just for the sake of having a part of speech that gives the politeness level, without meaning ‘to be’. This is why it is called the copula.
For instance, when someone is in a tea shop with a friend and the waiter ask them what they want to drink, after the friend says ‘お茶をください’ then the other can say ‘私は紅茶です’. Here it obviously doesn’t mean ‘i am a tea’, but です is just here to exhibit the politeness level because a noun can’t do that. It has no meaning.