Learn Hardcore Indonesian: Teman saya berdiri di depan gerbang sekolah. - My friend stands in front of the school gate.

To make it clearly present/ongoing, add an aspect marker or time word: Teman saya sedang berdiri di depan gerbang sekolah (sekarang).
To make it clearly past, add a past-time word/aspect: Tadi teman saya berdiri di depan gerbang sekolah, or Teman saya sudah berdiri di depan gerbang sekolah.

How do I say is standing right now?

Use the progressive marker sedang (neutral) or lagi (informal, very common) before the verb:

Teman saya sedang berdiri di depan gerbang sekolah.
Teman saya lagi berdiri di depan gerbang sekolah.

How do I say stood (earlier/already/just)?

Use time words or perfective markers:

Earlier: Tadi teman saya berdiri di depan gerbang sekolah.
Already (completed): Teman saya sudah/telah berdiri di depan gerbang sekolah.
Just now: Teman saya baru saja/barusan berdiri di depan gerbang sekolah.

Why is there no word for is? Why not use adalah?

You don’t need a copula. Berdiri is the verb. Adalah is only used for equating nouns (and sometimes for emphasis in formal writing), not with verbs or adjectives.

Correct: Teman saya berdiri…
Correct equational: Dia adalah guru.
Wrong here: Teman saya adalah berdiri…

Is the word order fixed? Can I front the place phrase?

Default order is Subject–Verb–Place: Teman saya berdiri di depan gerbang sekolah. You can front the place for emphasis or style, especially in writing:

Di depan gerbang sekolah, teman saya berdiri. Meaning stays the same; the fronted element gets focus.

What’s the difference between di, ke, and dari here?

di = at/in/on (static location): di depan gerbang sekolah.
ke = to/toward (motion): berjalan ke depan gerbang sekolah.
dari = from (source): datang dari depan gerbang sekolah.

Is there any nuance between di depan and di hadapan?

Both mean in front of, but:

di depan is neutral and very common for physical location.
di hadapan is more formal or emphasizes being in someone’s presence/facing someone: berbicara di hadapan kepala sekolah. For objects, di hadapan gerbang is grammatical but sounds more formal/literary than di depan gerbang.

Should I say di depan gerbang or di gerbang?

Both are possible but slightly different:

di gerbang = at the gate (at/near the gate itself).
di depan gerbang = in front of the gate (usually outside/facing the gate). Choose based on the spatial nuance you want.

How does gerbang sekolah work? Why not sekolah gerbang?

Indonesian noun–noun compounds put the head first, modifier second. Gerbang sekolah literally school gate (gate of the school). Sekolah gerbang is wrong. Similar patterns: pintu kamar (room door), tas sekolah (school bag).

How do I make the gate clearly definite (the school gate) or indefinite (a school gate)?

Indonesian has no articles, so use determiners if needed:

Definite: gerbang sekolah itu/tersebut (that/the aforementioned school gate), or gerbang sekolahnya (the school’s gate/its school gate; also marks definiteness).
Indefinite: sebuah gerbang sekolah (a school gate). Examples:
Teman saya berdiri di depan gerbang sekolah itu.
Teman saya berdiri di depan sebuah gerbang sekolah.

What’s the difference between teman saya, temanku, and teman aku?

All can mean my friend, with register differences:

teman saya: neutral–polite/formal.
temanku: informal; -ku is a clitic written attached.
teman aku: informal speech; in careful writing many prefer temanku. Regional/informal variants for I include aku, gua/gue (Jakarta), saya (formal). Colloquial spelling temen is common in speech; standard is teman.

Is teman saya singular or plural?

It’s number-neutral; context decides. To make it explicit:

Singular human: seorang teman saya (one friend of mine).
One of my friends: salah satu teman saya.
Plural: teman-teman saya, beberapa teman saya, dua/ tiga teman saya.
Formal plural for people: para teman saya (more written than spoken).

Why use berdiri? Could I use ada or berada instead?

berdiri emphasizes posture (standing upright).
ada means exist/be present; with places, it’s used for presence: Teman saya ada di depan gerbang sekolah (my friend is there, at that location).
berada is a more formal synonym of ada for location: Teman saya berada di depan gerbang sekolah. Use berdiri if posture matters; ada/berada if you just mean is located there.

What does the ber- in berdiri do? What’s the root?

Root: diri (self). The prefix ber- often makes intransitive verbs meaning to have/do/be in a state. berdiri literally be in a standing state. Note another common sense: for institutions, berdiri = be founded/established: Perusahaan itu berdiri pada tahun 1990.

How can I say in front of him/her/it?

Use the enclitic -nya on the location noun:

di depannya = in front of him/her/it. You can also say di depan dia (informal). Don’t say di depannya gerbang; that’s ungrammatical. If you need both, rephrase: di depan gerbang itu or di depannya, yaitu gerbang sekolah (in front of it, namely the school gate).

Any quick pronunciation tips for this sentence?

teman ≈ tə-man (unstressed e like a schwa).
saya = sa-ya (y as in yes).
berdiri = bər-di-ri (rolled or tapped r).
di depan = di də-pan (e in depan is a schwa).
gerbang = gər-bang (ng is a single velar nasal, like singer).
sekolah = sə-ko-lah (final h is audible).