Bugün dolmuş çok kalabalıktı, bu yüzden yürüdüm.

Dolmuş is a very common Turkish word for a kind of shared public transport, usually a shared minibus or sometimes a shared taxi.

It does not mean an ordinary private car or a standard city bus. A dolmuş usually follows a set route, and passengers get on and off along the way.

The word comes from dolmak, meaning to fill, because traditionally it would leave when it was full.

Turkish often drops subject pronouns when the verb already makes the subject clear.

Yürüdüm already means I walked:

• yürü- = walk
• -dü- = past tense
• -m = I

So adding ben is usually unnecessary unless you want emphasis:

• Ben yürüdüm. = I walked.
  This sounds more emphatic or contrastive.

In your sentence, the subject is understood naturally from yürüdüm.

Kalabalıktı is:

• kalabalık = crowded
• -tı = was

So kalabalıktı means it was crowded.

This is a very common Turkish pattern. Turkish often uses an adjective or noun directly, and then adds a form of to be when needed.

Compare:

• Dolmuş çok kalabalık. = The dolmuş is very crowded.
• Dolmuş çok kalabalıktı. = The dolmuş was very crowded.

So instead of a separate word like English was, Turkish adds it as an ending.

This is because of consonant harmony.

The past/copular ending can appear in forms like:

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

Which one you use depends on:

1. the last vowel of the word, and
2. whether the last consonant is voiced or voiceless.

In kalabalık, the last consonant is k, which is voiceless, so the ending begins with t, not d.

Also, the last vowel in kalabalık is ı, so the vowel in the ending is ı.

That gives you:

• kalabalık + tı = kalabalıktı

Here, çok means very because it is modifying the adjective kalabalık.

So:

• çok kalabalık = very crowded

This is important because çok can also mean many or a lot of when used with nouns.

Compare:

• çok kalabalık = very crowded
• çok insan = many people

So in your sentence, çok is acting like very, not many.

Bu yüzden means something like:

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

It connects the first idea to the result:

• the dolmuş was very crowded
• therefore / so I walked

It is very natural and common in Turkish.

A very similar expression is o yüzden, which is also extremely common in everyday speech. In many contexts, both work, though o yüzden can sound a little more conversational.

It comes from the verb yürümek, meaning to walk.

Step by step:

• yürümek = infinitive, to walk
• remove -mek → yürü-
• add past tense ending → -dü
• add I ending → -m

So:

• yürü + dü + m = yürüdüm

This means I walked.

The vowel ü appears because of vowel harmony: the last vowel of the stem yürü- is ü, so the past ending matches it.

Putting Bugün at the beginning is very natural in Turkish.

Time expressions often come early in the sentence to set the scene:

• Bugün dolmuş çok kalabalıktı, bu yüzden yürüdüm.

This is similar to English starting with Today, ...

Turkish word order is flexible, so you could also say:

• Dolmuş bugün çok kalabalıktı...

But starting with Bugün makes today the topic right away, which sounds very natural.

Because Turkish usually expresses to be differently from English.

In the present tense, Turkish often has no separate word for is/am/are:

• Dolmuş kalabalık. = The dolmuş is crowded.

In the past, Turkish adds an ending:

• Dolmuş kalabalıktı. = The dolmuş was crowded.

So English uses a separate word:

• was

But Turkish often uses:

• adjective/noun + ending

This is one of the basic structural differences between English and Turkish.

The comma is helpful, but in casual writing it is not always absolutely essential.

Here it separates:

1. the reason: Bugün dolmuş çok kalabalıktı
2. the result: bu yüzden yürüdüm

So the comma makes the sentence easier to read:

• Bugün dolmuş çok kalabalıktı, bu yüzden yürüdüm.

Without the comma, many people would still understand it, but the written pause is clearer with it.

Yes. Turkish often allows several natural variations.

For example:

• Bugün dolmuş çok kalabalıktı, o yüzden yürüdüm.
• Dolmuş bugün çok kalabalıktı, bu yüzden yürüdüm.
• Bugün dolmuş kalabalıktı, bu yüzden yürüdüm.

These all sound natural, though the focus changes slightly.

Your original sentence is completely normal and idiomatic. It is a good example of everyday Turkish.