Sahile gitmeden önce mayomu çantama koyuyorum.

Because gitmek (to go) usually takes the dative case when you say where someone is going.

• sahil = beach / shore
• sahile = to the beach

The ending -e / -a is the dative ending, often meaning to or toward.

So:

• sahile gitmek = to go to the beach

Gitmeden önce means before going or more literally before (one) goes.

It breaks down like this:

• gitmek = to go
• gitmeden = without going / before going in this kind of structure
• önce = before

Together, gitmeden önce is a very common Turkish pattern meaning:

• before doing something

Examples:

• yemek yemeden önce = before eating
• uyumadan önce = before sleeping
• evden çıkmadan önce = before leaving the house

So sahile gitmeden önce = before going to the beach.

Good question. The -me / -ma part is the same form used in negation, but in the pattern -meden önce, it does not simply mean a plain negative sentence like does not go.

In this structure:

• gitmeden önce = before going
• literally something like before not-yet-going

This is just how Turkish forms this expression. So you should learn -meden önce as a set pattern meaning:

• before doing ...

Similarly:

• çıkmadan önce = before leaving
• konuşmadan önce = before speaking

Mayomu contains two things:

• mayo = swimsuit
• -m = my
• -u = accusative case

So:

• mayom = my swimsuit
• mayomu = my swimsuit as a specific direct object

In this sentence, the speaker is putting a specific swimsuit into the bag, so Turkish uses the accusative.

Breakdown:

• mayo → mayom = my swimsuit
• mayom → mayomu = my swimsuit (as the object)

Because koymak (to put) has a direct object here, and that object is specific/definite.

In Turkish, definite direct objects usually take the accusative ending.

• mayo koyuyorum could sound more like I’m putting a swimsuit
• mayomu koyuyorum = I’m putting my swimsuit

Since my swimsuit is definite and specific, -u is used.

This is one of the most important Turkish patterns:

• indefinite object → often no accusative
• definite/specific object → accusative

Çantama also has more than one ending:

• çanta = bag
• -m = my
• -a = to / into (dative case)

So:

• çantam = my bag
• çantama = to my bag / into my bag

In English we say into my bag, but Turkish often uses the dative ending for the destination after verbs like koymak.

Because koymak usually marks the destination with the dative.

You are putting something into / onto / to somewhere:

• kitabı masaya koydum = I put the book on the table
• anahtarı cebime koydum = I put the key in my pocket
• mayomu çantama koyuyorum = I put my swimsuit in my bag

So the thing being moved goes in the accusative if definite (mayomu), and the place it is moved to goes in the dative (çantama).

Koyuyorum comes from koymak (to put).

Breakdown:

• koy- = verb stem
• -uyor = present continuous
• -um = I

So:

• koyuyorum = I am putting / I put

In Turkish, the -yor form is often used not only for something happening right now, but also for:

• habits
• routines
• things someone commonly does
• near-future actions in context

So this sentence can mean something like:

• I put my swimsuit in my bag before going to the beach or
• I’m putting my swimsuit in my bag before going to the beach

depending on context.

Yes, but it would change the feel.

• koyuyorum often sounds more immediate, natural in everyday speech, or like part of a routine being described.
• koyarım is the aorist and can sound more like I usually put or I put as a general habit.

So:

• Sahile gitmeden önce mayomu çantama koyuyorum. = natural everyday statement, often I put / I’m putting my swimsuit in my bag before going to the beach.

• Sahile gitmeden önce mayomu çantama koyarım. = more like Before going to the beach, I usually put my swimsuit in my bag.

Both can work, but -yor is very common in spoken Turkish.

Because Turkish normally places the main verb at the end.

This is one of the biggest differences from English.

English:

• I put my swimsuit in my bag before going to the beach.

Turkish:

• Sahile gitmeden önce mayomu çantama koyuyorum.
• literally: To-the-beach before-going my-swimsuit into-my-bag I-am-putting

Turkish word order is flexible, but the verb very often stays at the end.

Yes, Turkish word order is fairly flexible, especially for emphasis, but some orders sound more natural than others.

The given sentence is very natural:

• Sahile gitmeden önce mayomu çantama koyuyorum.

You could also say:

• Mayomu sahile gitmeden önce çantama koyuyorum.
• Mayomu çantama sahile gitmeden önce koyuyorum.

These are grammatical, but the emphasis changes a little.

General tendency in Turkish:

• time expressions often come early
• the verb usually comes last

So the original sentence is a very normal default order.

That y is a buffer consonant.

The noun mayo ends in a vowel. When Turkish adds another vowel-initial ending, it often inserts y to make pronunciation easier.

So:

• mayo + u would be awkward
• Turkish uses mayo + y + u

With the possessive included:

• mayo + m + u becomes mayomu

You will often see buffer consonants in Turkish, especially y, n, s, and sometimes others depending on the structure.

Because in Turkish, önce comes after the action it refers to.

So Turkish says:

• gitmeden önce
• literally: before going

Similarly:

• yemekten önce = before the meal
• gitmeden önce = before going
• uyumadan önce = before sleeping

This is just the normal Turkish structure.

Yes, functionally it does.

A very literal breakdown might feel unusual in English, but the natural meaning is simply:

• before going to the beach

So even if the Turkish structure looks different, you should understand it as the normal way to say that idea.

Here it is:

• Sahile = to the beach
• gitmeden önce = before going
• mayomu = my swimsuit (definite object)
• çantama = into my bag / to my bag
• koyuyorum = I put / I am putting

So the structure is:

• Before going to the beach, I put my swimsuit into my bag.