Learn Hardcore Turkish: Toplantıya daha uyumlu bir kıyafet giysem iyi olacak. - It will be better if I wear a more fitting outfit to the meeting.

Toplantıya = dative case (-a/-e): “to/for the meeting,” goal or purpose-oriented. It’s like saying “for the meeting” without using için.
Toplantıda = locative case (-da/-de): “at/in the meeting,” emphasizing location/time of the action.
Toplantı için = “for the meeting,” explicit purpose marker. All three can be correct, but they emphasize slightly different things (goal vs location vs explicit purpose).

Why is giysem used here? What does the ending -sAm mean?

Giysem is the conditional mood: giy- (wear) + -se/-sa (if) + -m (1st person). It literally means “if I wear.” The pattern -sAm iyi olur/olacak is a common, soft self-suggestion: “It would be good if I … / It’ll be good if I …,” roughly “I’d better …” or “I should probably …”

What nuance does iyi olacak add compared to iyi olur or iyi olurdu?

iyi olacak: future-looking, sounds like a plan/decision (“It will be good [if I do this]”).
iyi olur: more general/neutral recommendation (“It would be good”).
iyi olurdu: more hypothetical/softer; also used for counterfactuals depending on context (“It would have been good / It would be good, hypothetically”).

Could I use uygun instead of uyumlu? What’s the difference?

Uygun = “suitable/appropriate (for a purpose or rules).” For meetings, daha uygun is very idiomatic.
Uyumlu = “harmonious/compatible/matching (with the setting/style).” Daha uyumlu suggests something that fits the vibe/setting. Your sentence is fine, but daha uygun is slightly more typical for etiquette/suitability.

Is bir necessary in bir kıyafet? What changes if I drop it?

Bir is optional.

With bir: “a (single) outfit,” mildly singling out one unspecified outfit.
Without bir: “more appropriate outfit/clothing” in a generic sense (more like “more appropriate attire”). Both are natural here.

Why doesn’t kıyafet have the accusative (-i) ending? When would kıyafeti be used?

Indefinite direct objects in Turkish stay unmarked: kıyafet = “an outfit/clothes” (unspecified). You’d use kıyafeti when the outfit is definite/specific: Toplantıya o kıyafeti giysem… = “If I wore that outfit to the meeting…”

Can I say giyersem instead of giysem?

You can, but nuance differs:

giysem is the simple conditional used in suggestions: giysem iyi olur/olacak feels like “Maybe I should wear…”
giyersem (aorist + conditional) sets up a more straightforward condition-result: Bunu giyersem, daha ciddi görünürüm (“If I wear this, I look more serious”). For soft self-advice, giysem is the go-to.

What about giyinsem? Is that okay here?

Giyinmek = “to get dressed (in general).”

Daha uyumlu giyinsem = “if I dressed more appropriately” (general manner).
With a specific noun like kıyafet, use giymek: daha uyumlu bir kıyafet giysem. So your sentence correctly uses giymek.

Where is the subject “I”? Do I need to add ben?

It’s inside the verb. The ending -m in giysem marks 1st person singular. Adding ben is optional and used for emphasis: Ben toplantıya… giysem… (I, as opposed to someone else).

Can I change the word order?

Turkish is flexible, but the given order is the most natural. Alternatives:

Toplantıya, daha uyumlu bir kıyafet giysem, iyi olacak. (adding commas for clarity)
Daha uyumlu bir kıyafet, toplantıya giysem, iyi olacak. (possible but less neutral) Keep the verb near the end, and keep the adjective before its noun. Don’t move daha away from uyumlu.

What exactly does daha modify here? Do I need a “than” phrase?

Daha modifies uyumlu: daha uyumlu = “more appropriate/compatible.” Turkish doesn’t require an explicit “than X”; context supplies the baseline (e.g., than what I’d otherwise wear).

Could I say Toplantı için daha uygun bir kıyafet giymeliyim? How does that differ?

Yes. Giymeliyim (necessitative) = “I should/must wear,” a stronger, more direct commitment. Your original giysem iyi olacak is softer and more tentative. Both are correct; choose based on how strong you want the self-advice to sound.

What about using a nominalized clause like giymem: Toplantıya daha uygun bir kıyafet giymem iyi olacak?

That’s also correct. Giymem (my wearing) makes the subject explicit as a noun phrase: “My wearing a more suitable outfit to the meeting will be good.” It’s slightly more formal/plan-like than giysem, which feels like a spontaneous suggestion to oneself.

Any quick morphology breakdown of the whole sentence?

Toplantı-ya (meeting + to/for)
daha uyumlu (more compatible/appropriate)
bir kıyafet (an outfit)
giy-se-m (wear + if + I)
iyi ol-acak (good + will be)