Soğukta kalınca yanakların kızarabiliyor, bu yüzden içeri gir.

Soğukta means in the cold or out in the cold.

It is made from:

• soğuk = cold
• -da/-de/-ta/-te = the locative ending, meaning in / at / on

Here it becomes -ta, not -da, because Turkish changes this suffix to match the final consonant of the word. Since soğuk ends with a voiceless consonant sound, the suffix uses t instead of d.

So:

• evde = in the house
• okulda = at school
• soğukta = in the cold

Kalınca comes from kalmak, which usually means to stay, to remain, or sometimes to be left.

The ending -ınca / -ince / -unca / -ünce often means:

• when
• once
• sometimes if, depending on context

So soğukta kalınca means something like:

• when you stay in the cold
• if you stay out in the cold
• after being in the cold

In this sentence, it gives the situation that causes the next thing to happen.

Both are possible, but they feel slightly different.

• kalınca = when/if you stay
• kalırsan = if you stay

Kalınca often sounds more like a natural, expected result:
When you stay in the cold, your cheeks can get red.

Kalırsan sounds more directly conditional:
If you stay in the cold, your cheeks can get red.

So kalınca is a bit softer and more matter-of-fact here.

Turkish usually shows possession with a suffix attached to the noun.

Yanakların breaks down as:

• yanak = cheek
• -lar = plural
• -ın = your

So yanakların = your cheeks

This is very normal in Turkish, especially with body parts. Turkish often uses a possessive suffix where English uses a separate word like my, your, or his.

Compare:

• elim = my hand
• gözlerin = your eyes
• yanakların = your cheeks

Kızarabiliyor can be broken into parts:

• kızar- = become red / turn red
• -abil- = can / be able to
• -iyor = present continuous-style ending

So kızarabiliyor means:

• can turn red
• may get red
• is able to become red

In natural English here, it means that this is something that can happen.

Also, the verb is from kızarmak, which means to become red, to blush, or to redden.

Not really. In this sentence, -abil- does not mean a learned skill like I can swim.

Here it shows possibility or tendency:

• yanakların kızarabiliyor = your cheeks can get red / may get red

So it means this is something that happens under those conditions, not that your cheeks have a special ability.

This is a very common Turkish pattern.

With non-human plural subjects, Turkish often uses a singular verb. Since cheeks are not human, kızarabiliyor sounds natural.

So Turkish often prefers:

• Yanakların kızarabiliyor.

rather than:

• Yanakların kızarabiliyorlar.

The plural verb form is usually unnecessary here and sounds less natural.

Bu yüzden means:

• therefore
• so
• for this reason
• because of this

It introduces the result of what came before.

So the sentence structure is:

1. staying in the cold can make your cheeks turn red
2. for that reason, go inside

By contrast, çünkü means because and introduces the reason.

Compare:

• Soğukta kalınca yanakların kızarabiliyor, bu yüzden içeri gir.
  = Your cheeks can get red in the cold, so go inside.

• İçeri gir, çünkü soğukta kalınca yanakların kızarabiliyor.
  = Go inside, because your cheeks can get red in the cold.

İçeri means inside in the sense of toward the inside.

It is directional, so it fits with a verb of movement like gir.

So:

• içeri gir = go in / come in / go inside

A useful contrast is:

• içeri = inside, inward, to the inside
• içeride = inside, indoors, on the inside

For example:

• İçeri gir. = Go inside.
• İçeride bekle. = Wait inside.

Yes, girmek already means to enter / go in, so içeri is not absolutely required for basic meaning.

But Turkish very often uses direction words together with movement verbs for clarity and naturalness.

So:

• Gir. = Enter / Go in.
• İçeri gir. = Go inside / Come in.

Adding içeri makes the direction more explicit and sounds very natural in everyday speech.

Gir is the informal singular imperative of girmek.

It is used when speaking to:

• one person
• in a casual, familiar, or neutral way

So it corresponds to go in, come in, or get inside.

If you wanted to be more polite or address more than one person, you would usually say:

• girin = go in / come in

So:

• İçeri gir. = informal singular
• İçeri girin. = polite singular or plural

Turkish often leaves pronouns out when they are already clear from context or verb forms.

In this sentence:

• yanakların already shows your cheeks
• gir is a command directed at you

So adding sen is unnecessary.

If you said Sen içeri gir, it would sound more emphatic, as if you were stressing you specifically. Normally, Turkish prefers the shorter version unless emphasis is needed.