Dün arkadaşlarla çok eğlendik.

Dün means yesterday.

In Turkish, time words often come near the beginning of the sentence, so Dün arkadaşlarla çok eğlendik is a very natural order:
Yesterday, we had a lot of fun with friends.

That said, Turkish word order is flexible. You could move dün for emphasis, but sentence-initial position is the most neutral here.

In Turkish, with is often expressed by the suffix -la / -le attached directly to a noun.

So:

• arkadaş = friend
• arkadaşlar = friends
• arkadaşlarla = with friends

This is basically:

• arkadaşlar
  • -la → arkadaşlarla

There is also a separate form ile, which also means with. So arkadaşlarla and arkadaşlar ile mean the same thing, but the attached suffix form is very common in everyday Turkish.

This is because of vowel harmony.

The ending for with is -la or -le, depending on the last vowel of the word it attaches to.

• If the last vowel is a back vowel like a, ı, o, u, you use -la
• If the last vowel is a front vowel like e, i, ö, ü, you use -le

Here the word is arkadaşlar, whose last vowel is a, so the correct form is -la:

• arkadaşlar + la → arkadaşlarla

It can mean either, depending on context.

Turkish does not have articles like a and the in the same way English does. So arkadaşlarla can be understood as:

• with friends
• with the friends

In many contexts, English would naturally translate it as with friends.

If Turkish wants to be more specific, it usually does so through context or other words, not through an article.

Eğlendik breaks down like this:

• eğlen- = have fun / enjoy oneself
• -di- = past tense
• -k = we

So:

• eğlen-dik = we had fun

This is a very common Turkish pattern:
verb stem + past tense + person ending

Turkish often leaves out subject pronouns because the verb ending already tells you the person.

In eğlendik, the -k ending shows that the subject is we.

So:

• eğlendik = we had fun
• biz eğlendik = we had fun, but with extra emphasis on we

Using biz is possible, but it is not necessary unless you want contrast or emphasis.

The dictionary form is eğlenmek.

Turkish dictionaries list verbs in the -mek / -mak infinitive form. So:

• eğlenmek = to have fun / to enjoy oneself

From that verb, you get:

• eğlendim = I had fun
• eğlendik = we had fun
• eğleniyor = he/she is having fun
• eğleneceğiz = we will have fun

Because çok is acting as an adverb here, meaning a lot or very much.

It modifies the verb eğlendik, so çok eğlendik means:

• we had a lot of fun
• we enjoyed ourselves a lot

In Turkish, adverbs often come before the word they modify, so çok naturally comes before the verb.

Eğlenmek usually means to have fun, to enjoy oneself, or to be entertained.

So çok eğlendik is closer to:

• we had a lot of fun
• we really enjoyed ourselves

It is not exactly the same as English enjoy in every situation. For example:

• Filmi çok beğendim = I liked the film a lot
• Filmde çok eğlendim = I had a lot of fun during the film

A related verb is eğlendirmek, which means to entertain someone.
So:

• eğlenmek = to have fun
• eğlendirmek = to entertain

Yes. Turkish word order is flexible, though some orders sound more neutral than others.

The most neutral version here is:

• Dün arkadaşlarla çok eğlendik.

You could also hear:

• Arkadaşlarla dün çok eğlendik.
• Dün çok eğlendik arkadaşlarla.
  This can sound more conversational or marked.

In Turkish, the verb often comes at the end, and the words before it can move around depending on emphasis.

The letter ğ in Turkish is called yumuşak g, or soft g. It usually is not pronounced like a hard English g.

In eğlendik, it does not sound like egg-lendik. Instead, it softens or glides the sound. Many learners hear something roughly like:

• eylendik or
• a lengthened vowel before l

You do not need to force a strong consonant there. A soft, smooth pronunciation is best.

Yes. That would simply mean:

• Yesterday we had fun with friends.

Adding çok makes it stronger:

• Dün arkadaşlarla eğlendik. = We had fun yesterday with friends.
• Dün arkadaşlarla çok eğlendik. = We had a lot of fun yesterday with friends.

So çok adds intensity, but the sentence is complete without it.