Ben akşam yürüyüşünden sonra bir mandalina yiyorum.

Yes, it can be omitted.

In Turkish, the verb ending already shows the subject. Yiyorum already means I am eating, so ben is not grammatically necessary.

• Ben akşam yürüyüşünden sonra bir mandalina yiyorum.
• Akşam yürüyüşünden sonra bir mandalina yiyorum.

Both are correct. Adding ben usually makes I more explicit, often for emphasis, contrast, or clarity.

Akşam yürüyüşü means evening walk.

It is a Turkish noun compound:

• akşam = evening
• yürüyüş = walk
• akşam yürüyüşü = evening walk

This is a very common Turkish pattern: one noun describes another noun, and the second noun takes a special ending.

Other examples:

• okul kapısı = school door
• yaz tatili = summer holiday
• elma suyu = apple juice

So akşam yürüyüşü is not literally walk of evening in English style, but that is roughly how the structure works.

That -ü is part of the noun compound structure.

In Turkish, when one noun modifies another noun, the second noun usually takes what learners often call the compound suffix or third-person possessive-looking suffix:

• akşam yürüyüş-ü
• elma su-yu
• okul kapı-sı

Even though this suffix looks like a possessive ending, in many compounds it does not mean his/her/its in the usual sense. It is simply how Turkish commonly forms this kind of compound noun.

So here, yürüyüşü does not mean his walk by itself. In akşam yürüyüşü, it just helps form the meaning evening walk.

Because two things are happening:

1. yürüyüşü already ends with the compound suffix -ü
2. The phrase needs the ending -den because of sonra

So the form is:

• yürüyüş
  • -ü
    • -n-
      • -den
• yürüyüşünden

The -n- is a buffer consonant. Turkish often inserts it between a possessive-type ending and a case ending to make pronunciation and grammar work smoothly.

So:

• akşam yürüyüşü = evening walk
• akşam yürüyüşünden sonra = after the evening walk

Because sonra is a postposition, not a preposition.

English uses prepositions:

• after the walk

Turkish often uses postpositions:

• yürüyüşten sonra
• literally: from the walk after

So in Turkish, the noun phrase comes first, and sonra comes after it.

A very useful pattern to remember is:

• -den / -dan sonra = after
• -den / -dan önce = before

Examples:

• yemekten sonra = after the meal
• okuldan sonra = after school
• dersten önce = before class

Because sonra normally combines with the ablative case in Turkish.

The ablative ending is:

• -den / -dan / -ten / -tan

So the pattern is:

• X-den sonra = after X

In this sentence:

• akşam yürüyüşünden sonra = after the evening walk

This is one of those patterns that is best learned as a unit. Instead of thinking of sonra alone, it helps to memorize:

• bir şeyden sonra = after something

Here bir can work like a/an in English, though it can also literally mean one depending on context.

So:

• bir mandalina = a tangerine / one tangerine

Turkish does not have a separate word exactly like the English article a/an, so bir often fills that role.

Also, Turkish sometimes leaves bir out when English would still use a/an:

• Mandalina yiyorum. = I am eating a tangerine / I am eating tangerines, depending on context
• Bir mandalina yiyorum. = I am eating a tangerine / one tangerine

With bir, the noun feels more clearly singular and indefinite.

Yiyorum comes from the verb yemek, meaning to eat.

The form breaks down like this:

• verb stem: ye-
• present continuous marker: -iyor
• first-person singular ending: understood in the form -um pattern, giving yiyorum

So:

• yiyorum = I am eating

Some related forms:

• yiyorsun = you are eating
• yiyor = he/she/it is eating
• yiyoruz = we are eating
• yiyorsunuz = you are eating
• yiyorlar = they are eating

The verb yemek is slightly irregular-looking here because the stem combines with -iyor as yi-yor.

Because Turkish normally puts the verb at or near the end.

A very common neutral Turkish word order is:

• Subject + time/place/other information + object + verb

So here:

• Ben = subject
• akşam yürüyüşünden sonra = time phrase
• bir mandalina = object
• yiyorum = verb

This final-verb pattern is one of the biggest differences from English.

That said, Turkish word order is flexible. You can move parts around for emphasis, but the verb often still stays at the end.

It depends on what meaning you want.

• yiyorum usually means I am eating and is the present continuous form
• yerim is the aorist and often means I eat, I usually eat, or I would eat

So the difference is roughly:

• Bir mandalina yiyorum. = I am eating a tangerine.
• Bir mandalina yerim. = I eat a tangerine / I usually eat a tangerine.

In real Turkish, -iyor can sometimes be used for a current routine or a repeated action in context, but its core meaning is still the ongoing present. So if the intended meaning is specifically I am eating, then yiyorum is the expected form.

It is natural, but in everyday speech people often simplify it if the subject is obvious.

Very common versions would be:

• Akşam yürüyüşünden sonra bir mandalina yiyorum.
• Akşam yürüyüşünden sonra mandalina yiyorum.

The first keeps bir and sounds specific: a tangerine.
The second is slightly more general and natural in casual speech: I eat/am eating tangerine(s) after my evening walk, depending on context.

So the full sentence with ben is correct, but Turkish often drops pronouns and sometimes drops bir too if the meaning is already clear.