Bu fikir dün aklıma geldi.

Literally, aklıma geldi means came to my mind.

So the whole sentence is literally:

This idea came to my mind yesterday.

In natural English, that often becomes This idea occurred to me yesterday or I thought of this idea yesterday.

The important thing is that Turkish often uses aklına gelmek / aklına düşmek for the idea of something suddenly coming into your mind.

Because aklıma is built from several parts:

• akıl = mind, reason
• aklım = my mind
• aklıma = to my mind

So:

• akıl → base noun
• -ım / -im / -um / -üm = my
• -a / -e = dative case, often meaning to

Here it is:

• akıl
• aklım = my mind
• aklıma = to my mind

That is why the expression means came to my mind.

This happens because of a common sound change in Turkish.

Some two-syllable words lose a vowel when a suffix is added. Akıl is one of them.

So:

• akıl
  becomes
• aklım / more standard spelling: aklım

Then with the dative:

• aklım + a → aklıma

This kind of vowel drop is very common and worth getting used to.

Other examples include:

• burun → burnum = my nose
• ağız → ağzım = my mouth

Because of vowel harmony.

The dative ending is either -a or -e depending on the last vowel of the word.

In aklım, the last vowel is ı, which is a back vowel, so the dative becomes -a:

• aklım
  • -a → aklıma

Compare:

• ev → eve = to the house
• okul → okula = to the school

Yes. Bu fikir is the subject.

The sentence structure is basically:

• Bu fikir = this idea
• dün = yesterday
• aklıma = to my mind
• geldi = came

So the subject is the thing that came: this idea.

That is different from English, where we often say I thought of this idea and make I the subject. In Turkish, the sentence is often framed from the idea’s point of view: the idea came to my mind.

Geldi is the past tense of gelmek, which means to come.

• gel- = come
• -di = past tense

So geldi means came.

In this expression, Turkish uses come in the same way English can say It came to me or The idea came to me.

Because Turkish expresses that meaning with aklıma.

Instead of saying to me, Turkish says to my mind:

• bana = to me
• aklıma = to my mind

For this specific idea, aklıma gelmek is the natural expression.

So while bana geldi literally means came to me, it would not normally be used for an idea occurred to me. For that, aklıma geldi is the usual phrasing.

Yes, that is a very natural English translation.

Even though the Turkish structure is literally This idea came to my mind yesterday, the natural English meaning is often:

• I thought of this idea yesterday
• This idea occurred to me yesterday
• This idea came to me yesterday

All of these can work depending on context.

They are related, but not the same.

• aklıma geldi = it came to my mind, it occurred to me
• düşündüm = I thought, I thought about it

So:

• Bu fikir aklıma geldi suggests the idea appeared in your mind
• Bu fikri düşündüm suggests you actively thought about the idea

This is an important difference:

• occurring to you is more sudden or spontaneous
• thinking about something is more deliberate

Turkish word order is flexible, but the unmarked order often puts time expressions before the verb.

So:

• Bu fikir dün aklıma geldi.

is very natural.

You could also hear:

• Dün bu fikir aklıma geldi.
• Bu fikir aklıma dün geldi.

These all make sense, but the emphasis changes a little.

The version you have is a normal, neutral sentence.

Yes, absolutely.

That version is also correct and natural.

Compare:

• Bu fikir dün aklıma geldi. = This idea came to my mind yesterday.
• Dün bu fikir aklıma geldi. = Yesterday, this idea came to my mind.

The second version puts a little more focus on yesterday by placing it first.

Bu means this.

So:

• bu fikir = this idea

It is a demonstrative, used to point to something close in context, conversation, or attention.

Compare:

• bu = this
• şu = that / this one there
• o = that

Turkish does not have articles like English a/an and the.

So fikir can mean:

• idea
• an idea
• the idea

The exact meaning depends on context.

In this sentence, bu already makes it definite:

• bu fikir = this idea

So no article is needed.

Yes, very commonly.

Examples:

• Aklıma geldi. = It came to my mind.
• Aklına geldi mi? = Did it come to your mind?
• Aklımıza gelmedi. = It did not come to our mind.
• Aklıma geliyor. = It is coming to my mind / I am remembering it.
• Aklıma gelmişti. = It had come to my mind.

You can change both the person and the tense.

For example:

• aklına = to your mind

• aklına geldi = it came to your mind

• aklımıza = to our mind
• aklımıza geldi = it came to our mind

Yes, that is a good idea.

While it is understandable word by word, it is also a very common set expression in Turkish.

Useful patterns to memorize:

• Aklıma geldi. = It occurred to me.
• Aklıma bir fikir geldi. = An idea occurred to me.
• Hiç aklıma gelmedi. = It never occurred to me.
• Bu aklıma geldi. = This came to mind.

Learning it as a chunk will help you sound more natural.