Kalemtıraşı çantamda unuttum, bu yüzden kalemi açamadım.

Those endings mark the accusative case, which Turkish often uses for specific direct objects.

Here, the speaker is talking about:

• a particular pencil sharpener → kalemtıraşı
• a particular pencil → kalemi

So:

• kalemtıraş = pencil sharpener

• kalemtıraşı = the/specific pencil sharpener, as the object

• kalem = pencil
• kalemi = the/specific pencil, as the object

The exact vowel in the ending changes because of vowel harmony.

The basic dictionary form is kalemtıraş.

In this sentence:

• kalemtıraş
  • -ı → kalemtıraşı

So the -ı is a suffix added in the sentence; it is not part of the basic word itself.

Also, kalemtıraş is normally written as one word in Turkish.

It breaks down like this:

• çanta = bag
• -m = my
• -da = in / at / on

So çantamda means in my bag.

Compare:

• çantada = in the bag / in a bag
• çantamda = in my bag

This is very typical Turkish: ownership and location can both be expressed inside a single word.

Because Turkish often leaves out subject pronouns when the verb ending already shows who the subject is.

Here:

• unuttum already means I forgot
• açamadım already means I couldn't sharpen/open

So ben is not necessary.

If you add ben, it usually gives emphasis:

• Ben unuttum = I forgot

Unuttum is the simple past, first person singular of unutmak = to forget.

A useful breakdown is:

• unut- = forget
• -tu- = past tense form here
• -m = I

So unuttum = I forgot.

The past tense suffix appears in different shapes such as -dı, -di, -du, -dü, -tı, -ti, -tu, -tü depending on vowel harmony and consonant harmony.

Because they mean different things:

• açmadım = I did not sharpen/open
• açamadım = I could not sharpen/open

So açamadım expresses inability, not just simple negation.

A good pattern to remember is:

• -ama- / -eme- often means cannot / could not

So here:

• açamadım = I wasn’t able to sharpen it

Yes. With pencils, Turkish commonly uses açmak.

For example:

• kalem açmak = to sharpen a pencil
• kalemin ucunu açmak = to sharpen the tip of a pencil

So even though açmak often means to open, in this context it naturally means to sharpen.

Bu yüzden means because of this, for this reason, or that’s why.

It connects the first clause to the result:

• forgot the sharpener in the bag
• therefore / that’s why couldn’t sharpen the pencil

It is a very common way to link cause and result in Turkish.

Turkish usually puts the verb at the end, so unuttum at the end is very natural.

But the order before the verb is flexible:

• Kalemtıraşı çantamda unuttum
• Çantamda kalemtıraşı unuttum

Both are possible. The difference is mostly emphasis:

• starting with kalemtıraşı highlights the object more
• starting with çantamda highlights the location more

The main thing to notice is that the verb usually stays at the end of the clause.

Both are in the simple past.

• unuttum = I forgot
• açamadım = I couldn’t sharpen/open

So the sentence describes a completed past situation:

1. the speaker forgot the sharpener,
2. and as a result, they couldn’t sharpen the pencil.