Learn Hardcore Turkish: Stajyer bugün erken geldi.

Bir stajyer bugün erken geldi. (an intern) To make it clearly specific, use a demonstrative or a possessive:
Bu/O stajyer bugün erken geldi. (this/that intern)
Stajyerimiz bugün erken geldi. (our intern)

Why are bugün and erken before the verb? Can I move them?

Neutral word order is Subject – Time – Manner – Verb, so Stajyer bugün erken geldi is the most natural. You can topicalize bugün by putting it first:

Bugün stajyer erken geldi. Keep time before manner; Stajyer erken bugün geldi sounds awkward. The element right before the verb tends to be in focus (here, erken).

What’s happening morphologically in geldi? How is past tense formed?

gel- (come) + -di (simple past). In simple past, 3rd singular has no extra personal ending: geldi = he/she/it came. With vowel harmony and voicing, the past suffix appears as -dı/-di/-du/-dü or -tı/-ti/-tu/-tü:

geldim (I came), geldin (you came), geldi, geldik, geldiniz, geldiler
After a voiceless consonant: git-ti (he/she went).

What’s the difference between geldi and gelmiş?

geldi: direct/witnessed past; the speaker knows or asserts it as a fact.
gelmiş: evidential/inferential; reported, heard, or inferred, sometimes with a nuance of surprise. Example: Stajyer bugün erken gelmiş. (Apparently/it seems the intern came early today.)

How do I negate it?

Insert the negative -me/-ma before the past:

Stajyer bugün erken gelmedi. (didn’t come early today)

How do I ask a yes/no question?

Use the question clitic mi/mı/mu/mü, written separately and harmonized to the preceding word:

Neutral: Stajyer bugün erken geldi mi?
To ask specifically about “early”: Stajyer bugün erken mi geldi? Negative: Stajyer bugün erken gelmedi mi?

Can I drop the subject?

Yes. Turkish often omits subjects when clear from context:

Bugün erken geldi. (He/She came early today.)

How do I say “The interns came early today”? Does the verb have to be plural?

Pluralize the noun with -ler/-lar:

Stajyerler bugün erken geldi.
Stajyerler bugün erken geldiler. With an overt human plural subject, both singular and plural verb are acceptable; the plural verb is more explicit/emphatic. If there is no overt subject, use the plural verb: Bugün erken geldiler.

Is erken an adjective or an adverb? What about erkenden?

erken is an adjective that also functions adverbially (no extra ending needed): erken geldi (came early).
erkenden means “early, early on,” often with a nuance of “nice and early/earlier than usual”:

Erkenden geldik. (We came nice and early.)

Pronunciation tips for stajyer, bugün, geldi?

stajyer: the j is like the s in English “measure” (zh); y as in “yes”; final r is a tapped r.
bugün: ü is a front rounded vowel (like French u/German ü); stress commonly on the last syllable: bu-GÜN.
geldi: both vowels are clear (not schwa), stress usually on the last syllable: gel-Dİ.

How do I say “Our intern came early today”?

Add a possessive suffix to stajyer:

Stajyerimiz bugün erken geldi.

How do I add a place (e.g., “to the office”)?

Use the dative -e/-a for destinations:

Stajyer bugün ofise erken geldi.
Stajyer bugün okula erken geldi.

Is bugün one word or two?

One word: bugün. Related forms include bugünler (these days) and bugünlerde (these days, lately).

How do I say “earlier than usual”?

Use a comparative:

Her zamankinden daha erken geldi.
Normalden daha erken geldi. You can also use erkenden for a general “early on” feel: Bugün erkenden geldi.

Does gelmek mean “to come” or “to arrive”? Could I use something else?

gelmek covers both “come” and “arrive” in many contexts. For formal “arrive,” Turkish also has varmak:

Bugün ofise erken vardık. (We arrived at the office early today.)

Is there a difference between “early” and “quickly” in Turkish?

Yes:

erken = early (time)
çabuk/hızlı = quickly/fast (speed) Compare: Erken geldim (I came early) vs Çabuk geldim (I came quickly).