Learn Hardcore Turkish: Siz unutmayın diye takvime küçük bir not düştüm. - I added a small note to the calendar so that you don’t forget.

Here, diye introduces a purpose clause and means “so that / in order that.” It links the reason/purpose to the main action. Other common uses of diye are:

quoting: Unutmayın diye not yazdı. = “He wrote ‘Don’t forget.’”
giving a reason one believed: Geç kalırsın diye aradım. = “I called thinking you might be late.”

Why is it unutmayın and not something like unutmayasınız?

unutmayın = negative imperative, 2nd person plural/formal (“don’t forget”).
unutmayasınız = older/very formal/archaising optative; you might see it in literature or very polite speech, but in everyday Turkish unutmayın is the norm.

Can I drop the pronoun Siz?

Yes. The verb ending -yın already encodes “you (plural/formal).” So you can say:

Unutmayın diye takvime küçük bir not düştüm. Keeping Siz adds emphasis or clarity about who is not supposed to forget (e.g., “you specifically” as opposed to someone else).

Why is it takvime and not takvimde?

takvime is dative “to/into the calendar,” matching the idea of adding/placing something onto the calendar.
takvimde is locative “in/on the calendar,” focusing on location. With verbs like yazmak/bırakmak you could hear takvimde, but the set phrase takvime not düşmek prefers the dative.

What does not düştüm mean literally? Why use düşmek with “note”?

Literally, düşmek is “to fall,” but not düşmek is a set expression meaning “to jot/enter a note (into a record/system/calendar).” It’s common in office/IT contexts:

Sisteme bir not düştüm. = “I left a note in the system.” Near-synonyms: not yazmak, not bırakmak, eklemek, kaydetmek (each with slightly different nuance).

Why is there no accusative suffix on not (why not notu)?

Indefinite direct objects in Turkish typically remain bare (no -ı/-i/-u/-ü): küçük bir not. If the note is specific/definite, you can mark it:

Sisteme şu notu düştüm. = “I entered that note into the system.” With the light-verb compound not düşmek, both bare and marked forms occur, depending on definiteness.

What’s the difference between using diye and using için for purpose?

Both can express purpose:

Siz unutmayın diye takvime küçük bir not düştüm. (colloquial/natural)
Unutmamanız için takvime küçük bir not düştüm. (slightly more formal/explicit) The için version uses a verbal noun: unut-ma-ma-nız
- için (“for your not forgetting”).

How would this pattern change with other persons (I/he/she)?

With diye, the verb in the purpose clause matches the subject:

Ben unutmayayım diye… (“so that I don’t forget”) [1st person optative -ayım]
O unutmasın diye… (“so that s/he doesn’t forget”) [3rd person optative -sın]
Sen unutma diye… (“so that you (sg) don’t forget”) [2nd sg imperative]
Siz unutmayın diye… (“so that you (pl/formal) don’t forget”) [2nd pl imperative]

Is the word order fixed? Could I say Küçük bir not takvime düştüm?

Default and most natural is indirect object + direct object + verb:

Takvime küçük bir not düştüm. Küçük bir not takvime düştüm is possible but sounds marked/less natural. You can also place the purpose clause last:
Takvime küçük bir not düştüm, siz unutmayın diye.

Does küçük bir not mean literally “a small note” or is it softening the tone?

Both are possible. Literally “a small note,” but like in English, küçük can soften tone and make it sound casual/polite (“just a little note”). Context decides whether size or politeness is foregrounded.

Could I say ajandaya instead of takvime?

Yes, with a nuance:

takvim = calendar (wall/phone/Google Calendar)
ajanda = planner/agenda/diary If you wrote in a personal planner, ajandaya is more idiomatic; for a digital or wall calendar, takvime fits well.

Quick morphology check: what are the parts of unutmayın and düştüm?

unut-mayın: root unut- (forget) + negation -ma-
- 2pl/formal imperative -yın (buffer y).
düş-tü-m: root düş-
- past -di → allomorph -tü (vowel harmony + devoicing after ş) + 1sg -m.

Is there any difference between saying Unutursunuz diye… and Unutmayın diye…?

Yes:

Unutmayın diye… = purpose (“so that you don’t forget”).
Unutursunuz diye… = speaker’s concern/expectation (“thinking you might forget / in case you forget”), often closer to “because I thought you might forget.” They’re related but not identical in nuance.