Dede sahilde yürümek istedi, ama anneannem evde kalmayı tercih etti.

Because they are in the locative case, which means in / at / on a place.

• sahil → sahilde = at/on the beach
• ev → evde = at home / in the house

Turkish usually expresses location with a suffix instead of a separate preposition. The locative suffix has four forms: -da, -de, -ta, -te.

Because different Turkish verbs take different kinds of complements.

• istemek usually takes the plain infinitive:
  yürümek istedi = wanted to walk
• tercih etmek usually takes a noun or a verbal noun/object:
  kalmayı tercih etti = preferred staying / preferred to stay

So this is not random. It is part of how these verbs are normally used.

kalmayı can be broken down like this:

• kal- = stay
• -ma = verbal noun marker
• -yı = accusative case
• -y- = buffer consonant

So kalmayı literally works like the staying as the object of tercih etti.

Turkish often turns a verb into a noun-like form this way when it needs to function as the object of another verb.

Because tercih etmek often takes a definite object, and here the object is the action staying at home.

So:

• evde kalmayı tercih etti = preferred staying at home

The -yı is the accusative ending. Since kalma ends in a vowel, Turkish inserts a buffer y before the accusative suffix.

They are both in the simple past tense.

• istedi = wanted
• tercih etti = preferred

They are also third person singular. In Turkish, the third-person-singular past form often has no extra personal ending beyond the tense marker, so the subject tells you who did the action.

Because tercih etmek is a compound verb:

• tercih = preference
• etmek = to do / to make

Together, tercih etmek means to prefer.

Turkish has many verbs like this:

• yardım etmek = to help
• fark etmek = to notice
• telefon etmek = to telephone

So tercih etti is simply the past-tense form of that compound verb.

This is because of Turkish consonant harmony in suffixes.

The past tense suffix appears as one of these forms:

• -dı, -di, -du, -dü
• -tı, -ti, -tu, -tü

After a voiceless consonant such as t, Turkish uses the t version. Since etmek has the stem et-, it becomes:

• et-ti = etti

So etdi would not be correct in standard Turkish.

anneannem includes the possessive ending -m, which means my:

• anneanne = maternal grandmother
• anneannem = my maternal grandmother

By contrast:

• dede = grandfather / grandpa

If you wanted to say my grandfather explicitly, you would usually say dedem.

That said, kinship words in Turkish are sometimes used like names or titles, so dede can still sound natural depending on context.

Turkish usually puts the main conjugated verb at the end of the clause.

So the structure here is roughly:

• Dede | sahilde | yürümek | istedi
• anneannem | evde | kalmayı | tercih etti

This is much more natural in Turkish than trying to match English word order. A learner should get used to the idea that the final verb often carries the core tense and meaning of the clause.

Because Turkish does not use a separate word like English to before the infinitive.

Instead, the infinitive is formed with -mek / -mak:

• yürümek = to walk
• kalmak = to stay

When Turkish needs a more noun-like form, it often uses -ma / -me, as in kalmayı.

So English to is not translated word-for-word here.

ama means but.

It connects two clauses and shows contrast:

• Dede sahilde yürümek istedi
• ama
• anneannem evde kalmayı tercih etti

So it works very much like English but.

They are close in meaning, but not identical.

• istemek = to want
• tercih etmek = to prefer

So:

• yürümek istedi = he wanted to walk
• kalmayı tercih etti = she preferred to stay

tercih etmek suggests a choice between options, while istemek is a more general expression of wanting.