Learn Hardcore Turkish: Dekontu saklayalım, o ödeme için gerekebilir. - Let’s keep the receipt; it might be necessary for that payment.

The -u is the definite accusative suffix, marking a specific, known direct object: dekont-u = the receipt.
Without it (dekont saklayalım), it means an indefinite object: let’s keep a receipt (some receipt, not a particular one).
Vowel harmony picks -u (not -i/-ı/-ü) because the last vowel in dekont is back and rounded (o).

What does saklayalım express grammatically? How is “let’s …” formed in Turkish?

sakla-yalım is the 1st person plural optative: a proposal/suggestion meaning let’s keep.
Form: verb stem + -(y)alım/elim. The vowel alternates with harmony; the buffer y appears if the stem ends in a vowel.
More examples: gidelim (let’s go), başlayalım (let’s start), konuşalım (let’s talk).

Why is there a y in saklayalım?

sakla ends with a vowel; the optative suffix -alım/elim begins with a vowel.
Turkish inserts the buffer consonant y to avoid hiatus: sakla + y + alım → saklayalım.

Does saklamak mean “to save” like a computer file?

saklamak: to keep/store/put away, often physically; also to hide.
For saving a file, use kaydetmek.
Other near-synonyms by nuance:
- tutmak: to hold/keep hold of.
- korumak: to protect.
- bir kenara koymak: to set aside.

Could I say Onu saklayalım instead?

Yes. onu = it (3rd person singular accusative).
Use onu when the referent is clear from context. Dekontu is clearer if you want to name the item explicitly.

What does o add in o ödeme için? Can I drop it or use bu/şu?

o marks a specific, previously mentioned or contextually distant item: o ödeme = that payment.
Dropping o (ödeme için) makes it more generic: for (the) payment in general or the one at hand.
bu = this (near the speaker), şu = that (near the listener or just mentioned). All are grammatical; they change the deictic nuance.

Why is it ödeme için and not ödemenin için or ödemeye?

The postposition için takes bare nouns: ödeme için (for the payment).
With personal pronouns, you use genitive forms: benim için, senin için, onun için.
ödemeye (dative) = to the payment, not right here. ödemede (locative) = at/during the payment, different meaning.

What exactly is gerekebilir, and how is it built?

From gerekmek (to be necessary).
gerek-e-bil-ir = may be necessary (potential mood -ebil- + aorist -ir, 3rd singular).
Negatives: gerekmez (it’s not necessary), gerekmeyebilir (it might not be necessary).
Here the understood subject is the receipt from the first clause.

Can I use lazım olabilir instead of gerekebilir? Any nuance?

Yes: lazım olabilir also means may be needed.
gerek- is a touch more formal/neutral; lazım is very common and colloquial.
You can also say: O ödeme için dekont lazım olabilir.

Is the comma here normal? Should I add çünkü?

Yes, Turkish often links a suggestion and its reason with a comma.
Adding çünkü makes the causal link explicit: Dekontu saklayalım, çünkü o ödeme için gerekebilir.
A semicolon also works stylistically.

Can I change the word order in the second clause?

The natural order is to put adverbials before the predicate: O ödeme için gerekebilir.
You can front the subject for clarity/emphasis: Dekont o ödeme için gerekebilir.
Gerekebilir o ödeme için is possible in speech for emphasis but sounds marked; default is to keep gerekebilir sentence-final.

Any pronunciation tips for words in this sentence?

ö in ödeme is like German ö or French eu in deux.
ç in için is like ch in chair.
Syllables: sak-la-ya-lım (stress typically near the end: sak-la-ya-LIM); ge-re-ke-bi-lir (ge-re-ke-bi-LİR).

What’s the difference among dekont, makbuz, fiş, and fatura?

How do I say “Shall we keep the receipt?” or “Let’s not keep the receipt”?