Meyve suyunu pipetle içiyorum.

Because meyve suyunu is the definite direct object of the sentence.

• meyve suyu = fruit juice
• meyve suyunu = the fruit juice / the fruit juice (as the thing being drunk)

In Turkish, a specific/definite direct object usually takes the accusative ending. Since the speaker is talking about a particular fruit juice, Turkish marks it.

So:

• Meyve suyu içiyorum = I’m drinking fruit juice / some fruit juice
• Meyve suyunu içiyorum = I’m drinking the fruit juice

This is because meyve suyu is a very common Turkish noun compound.

• meyve = fruit
• su = water
• meyve suyu = fruit juice

In this kind of compound, the second noun often takes a form that looks like a 3rd person possessive ending:

• su → suyu

So although it may look strange at first, meyve suyu is the normal dictionary form for fruit juice.

Similar examples:

• elma suyu = apple juice
• portakal suyu = orange juice

The n is a buffer consonant.

Here is what is happening:

• base compound: meyve suyu
• accusative ending: -(y)ı / -(y)i / -(y)u / -(y)ü
• because the word already ends in a possessive-type ending, Turkish often inserts n before certain case endings

So:

• meyve suyu
  • accusative → meyve suyunu

You do not need to think of this as a separate meaning. It is just part of how Turkish links endings smoothly.

Pipetle means with a straw or using a straw.

It comes from:

• pipet = straw
• ile = with
• pipet ile → pipetle

In everyday Turkish, ile often attaches to the noun and becomes:

• -la
• -le

depending on vowel harmony.

So these mean the same thing:

• pipet ile
• pipetle

The shorter attached form is very common.

Because of vowel harmony.

The last vowel in pipet is e, which is a front vowel, so the ending becomes -le.

Compare:

• pipetle = with a straw
• bardakla = with a glass
• kaşıkla = with a spoon

The vowel in the ending changes to match the word before it.

İçiyorum is the present continuous form, built like this:

• iç- = drink
• -iyor = present continuous
• -um = I

So:

• içiyorum = I am drinking

Depending on context, Turkish present continuous can sometimes also sound natural in English as a present action around now, but the most direct translation is I am drinking.

Because Turkish often drops subject pronouns when they are already clear from the verb ending.

• içiyorum already means I am drinking
• the -um ending shows the subject is I

So:

• Ben meyve suyunu pipetle içiyorum.
• Meyve suyunu pipetle içiyorum.

Both are possible, but the second is more natural unless you want emphasis.

Using ben would add emphasis, like:

• I’m the one drinking the fruit juice with a straw.

Because Turkish normally prefers Subject–Object–Verb word order, and the verb often comes last.

This sentence is structured like:

• Meyve suyunu = the fruit juice
• pipetle = with a straw
• içiyorum = I am drinking

So literally it is something like:

• The fruit juice with a straw am drinking

That sounds strange in English, but it is normal in Turkish.

Yes, Turkish word order is flexible, but changing it usually changes the focus or emphasis.

For example:

• Meyve suyunu pipetle içiyorum.
  Neutral: I’m drinking the fruit juice with a straw.

• Pipetle meyve suyunu içiyorum.
  Emphasizes with a straw a bit more.

• İçiyorum meyve suyunu pipetle.
  Much less neutral in everyday standard usage; possible in special contexts, but not the normal learner pattern.

For beginners, it is best to learn the neutral order with the verb at the end.

Yes. Pipet is a standard and very common word for drinking straw.

So:

• pipet = straw
• pipetle içmek = to drink with a straw

Just be aware that in other contexts, depending on region or situation, people may use other expressions too, but pipet is the normal everyday word learners should know.

You would usually drop the accusative ending:

• Meyve suyu pipetle içiyorum.

That gives a more indefinite/non-specific sense: I’m drinking fruit juice with a straw.

Compare:

• Meyve suyu içiyorum. = I’m drinking fruit juice.
• Meyve suyunu içiyorum. = I’m drinking the fruit juice.

This definite vs. indefinite object distinction is very important in Turkish.

A rough pronunciation guide is:

• Meyve ≈ MEY-veh
• suyunu ≈ soo-yoo-NOO
• pipetle ≈ pee-PET-leh
• içiyorum ≈ ee-chee-YO-room

A few helpful notes:

• ç sounds like ch
• y sounds like English y
• Turkish vowels are usually pronounced clearly and consistently
• stress in Turkish is often lighter and more even than in English, though compounds and endings can affect it

A rough full reading:

MEY-veh soo-yoo-NOO pee-PET-leh ee-chee-YO-room