Learn Hardcore Turkish: Denize gideceksen güneş kremini yanına almalısın. - If you go to the sea, you should take your sunscreen with you.

Denize = deniz (sea) + -e (dative “to”) → “to the sea”
gidecek = git- (go) + -ecek (future tense) → “will go”
-sen = second-person singular conditional (“if you …”)
Together Denize gideceksen literally means “if you will go to the sea,” i.e. “if you’re going to the sea.”

What’s the difference between the future conditional gideceksen and the simple conditional gidersen?

• gidersen = git- + -er (aorist/present style) + -sen → “if you go” (general or habitual condition)
• gideceksen = git- + -ecek (future) + -sen → “if you will go” (a condition that the speaker sees as a real future plan or prediction)
In practice, gideceksen implies you’ve already decided or are planning to go; gidersen is more neutral/general.

Why is güneş kremini in the accusative case (with -i)?

Turkish marks definite direct objects with the accusative suffix -ı/-i/-u/-ü. Here you’re referring to a specific sunscreen (“your sunscreen”), so you add -i to güneş kremi, yielding güneş kremini.

What does yanına mean, and why is it yanına instead of yanında?

• yanına = yan (side) + -ına (dative) → “to your side,” i.e. “with you” (movement)
• yanında = yan + -ında (locative) → “at your side” (location)
Since you’re telling someone to take the cream along, you use dative yanına (“to by your side”).

How is almalısın formed, and what nuance does it convey?

• al- = verb root “take”
• -malı/-meli = necessity suffix (“ought to,” “must,” “should”)
• -sın = second-person singular ending
So almalısın = “you should take” or “you must take.”

Could I express obligation differently, for example using zorunda or gerekiyor?

Yes. Alternative forms:

… yanına alman gerekiyor. (literally “you need to take … with you.”)
… yanına almak zorundasın. (literally “you are obliged to take … with you.”)
All convey obligation, but -malı is more direct “should/ought to,” while zorunda/gerekiyor feel a bit stronger or more formal.

Is the word order fixed? Can I swap the clauses?

Turkish is relatively flexible. You could say:
Güneş kremini yanına almalısın, denize gideceksen.
Both versions are correct, but the original order (if-clause first) is more natural in advising contexts. A comma before denize helps clarify the switch.

How do I make the conditional negative? (“if you are not going to the sea…”)

Insert the negative suffix -me/-ma before the tense/conditional:

gitme- (negated root) + -yecek (future) + -sen → gitmeyeceksen
So:
Denize gitmeyeceksen, … → “If you are not going to the sea, …”