Learn Hardcore Turkish: Bugünkü toplantı yorucu.

Because -ki here is a suffix, not the separate conjunction ki. As a suffix, it’s written together with the word it attaches to: bugünkü, dünkü, evdeki, etc. The standalone conjunction ki is written separately.

Why is it bugün+kü (bugünkü) and not bugün+ki or bugün+kı?

In this set phrase, the suffix surfaces as -kü. The common time words have fixed, conventional forms:

bugünkü, dünkü (with -kü)
but yarınki, şimdiki (with -ki) These are established standard forms you should memorize.

Can I say Bugünün toplantısı instead of Bugünkü toplantı?

Yes, it’s grammatical. Nuance:

Bugünkü toplantı is the most natural, compact way to say “today’s meeting.”
Bugünün toplantısı is a possessive construction (“the meeting of today”) and can sound more formal or contrastive, used when you want to emphasize “today’s (as opposed to another day’s).”

Is the sentence complete without a verb? Where is “is”?

Yes. Turkish has a “zero copula” in the present tense for 3rd person. Bugünkü toplantı yorucu literally “today’s meeting tiring” = “Today’s meeting is tiring.”

How do I say it in the past or future?

Add the copular/tense endings to the predicate adjective:

Past: Bugünkü toplantı yorucuydu (was tiring).
Reported past: yorucuymuş (was apparently tiring).
Future: yorucu olacak (will be tiring).
General/neutral statement: yorucudur (it is tiring, in a general/definitional or formal tone).

How do I negate it or make it a question?

Negation: Bugünkü toplantı yorucu değil (is not tiring). Past: yorucu değildi.
Yes–no question: Bugünkü toplantı yorucu mu? Past: yorucu muydu?

What’s the difference between yorucu, yorgun, yoruldum, and yoruyor?

yorucu = tiring (describes something that causes tiredness).
yorgun = tired (describes a person’s state).
yoruldum = I got tired / I’m tired (intransitive verb yorulmak).
yoruyor = is tiring / is making [someone] tired (present continuous of transitive yormak). For example: Bugünkü toplantı beni yoruyor = “Today’s meeting is tiring me.”

Where can I put intensifiers like “very”?

Before the predicate adjective: Bugünkü toplantı çok yorucu. You can also use oldukça, epey, inanılmaz for nuance: oldukça yorucuydu, inanılmaz yorucuydu, etc.

Do adjectives agree with the noun in number or gender?

No. Turkish adjectives don’t change for number or gender. Plural subject example: Bugünkü toplantılar yorucu. (“Today’s meetings are tiring.”)

Can I move words around? For example, start with the adjective?

The neutral order is Subject + Predicate: Bugünkü toplantı yorucu. You can front the predicate for emphasis: Yorucu bugünkü toplantı, but it sounds marked/poetic. Keep the standard order in normal speech.

Is toplantı “a meeting” or “the meeting”? There’s no article.

Turkish has no articles. Here, Bugünkü toplantı is specific by context (“today’s meeting”), so it corresponds to English “the meeting.”

How do I say “I’m tired from today’s meeting”?

Use the ablative -dan/-den:

Bugünkü toplantıdan yoruldum.
Bugünkü toplantıdan yorgunum. You can also say: Bugünkü toplantı yüzünden çok yoruldum (“because of today’s meeting”).

What’s going on morphologically with yorucu?

It’s the deverbal adjective suffix -(I)CI (four-way harmony: -ıcı/-ici/-ucu/-ücü) added to the verb root:

yor- (tire) + -ucu → yorucu (“tiring”). The linking vowel -u- appears by vowel harmony.

Any pronunciation tips?

Bugünkü: the ğ in bugün is silent/lengthens the preceding vowel; ü is a front rounded vowel (purse your lips).
toplantı: final ı is the undotted ı (like the “e” in “problem” for many English speakers, but central).
yorucu: c is pronounced like English “j” in “jam.”