İkisi de kütüphanede ders çalışıyor.

İkisi de means both of them.

Breakdown:

• iki = two
• ikisi = the two of them / the two ones
• de = also / too

Together, ikisi de gives the sense of both.

So this phrase is a very common way to say:

• both of them
• the two of them, too

In this sentence, it means both people are doing the action.

Because iki by itself only means two.

To say the two of them or those two, Turkish often uses iki + -(s)i:

• ikisi = the two of them / the second one? No—here it means the two ones, often understood as the two of them

This -si is a very common form in Turkish when a number stands in for a noun:

• biri = one of them / one person
• ikisi = two of them
• üçü = three of them

So ikisi de is the natural expression for both of them.

Here, de adds the meaning of also / too, but in the pattern ikisi de, the whole expression is understood as both.

Compare:

• Ali de geliyor. = Ali is coming too.
• İkisi de geliyor. = Both of them are coming.

So although de often literally means too/also, in combinations like ikisi de, üçü de, hepimiz de, it helps create meanings like:

• both
• all three
• all of us

Yes—there are two different forms that look similar:

1. de as a separate word = also / too
2. -de as a suffix = in / at / on

In this sentence, you actually have both:

• ikisi de → de is a separate word meaning also/too, contributing to both
• kütüphane-de → -de is the location suffix, meaning in the library

A useful rule:

• If it is the also/too word, it is written separately
• If it means in/at/on, it is attached as a suffix

Because Turkish adds the locative suffix to show location.

• kütüphane = library
• kütüphanede = in the library / at the library

The suffix here is -de, chosen by vowel harmony:

• after front vowels, you often get -de / -te
• after back vowels, you often get -da / -ta

So:

• evde = at home
• okulda = at school
• masada = on the table
• kütüphanede = in the library

Çalışmak can mean to work or to study, depending on context.

But Turkish very often uses ders çalışmak specifically for studying lessons/school subjects.

So:

• çalışıyor can mean is working or is studying
• ders çalışıyor clearly means is studying

This is a very common collocation in Turkish.

Examples:

• Ofiste çalışıyor. = He/She works in an office.
• Sınav için ders çalışıyor. = He/She is studying for an exam.

It is the present continuous tense.

Breakdown:

• çalış- = study / work
• -ıyor = present continuous marker

So çalışıyor means:

• is studying
• is working

depending on context.

This tense is used for actions happening:

• right now
• around this period
• sometimes for near-future or habitual situations in context

In this sentence, it most naturally means are studying.

Good question. In Turkish, the third-person plural ending -lar/-ler is often optional when the subject already clearly shows plurality.

So both of these can be correct:

• İkisi de kütüphanede ders çalışıyor.
• İkisi de kütüphanede ders çalışıyorlar.

The version without -lar is very common and natural.

Why? Because ikisi de already tells you that there are two people, so Turkish does not always need to mark plurality again on the verb.

This is normal with plural subjects in Turkish.

Yes, in a sense.

Turkish often does not need a separate subject pronoun such as onlar (they) if the subject is already clear.

Here, ikisi de already functions as the subject:

• İkisi de = both of them

So there is no need to say:

• Onlar, ikisi de... — this would be unnatural here

Turkish prefers the shorter, cleaner structure.

The basic order here is:

• İkisi de = subject
• kütüphanede = place
• ders çalışıyor = verb phrase

This is a very natural Turkish word order.

Turkish is flexible, so you can move parts around for emphasis. For example:

• Kütüphanede ikisi de ders çalışıyor.
  Emphasizes in the library
• Ders çalışıyor ikisi de kütüphanede.
  Possible in speech/poetic emphasis, but less neutral

The most neutral version is the original:

• İkisi de kütüphanede ders çalışıyor.

Yes. İkisi de can refer to people or things, depending on context.

Examples:

• İkisi de geldi. = Both of them came.
• Kitapların ikisi de pahalı. = Both of the books are expensive.

So the phrase itself is not limited to people. You understand what it refers to from the context.

Yes. A fuller version is:

• Her ikisi de

This also means both of them, but it can sound a little more explicit or formal than just ikisi de.

Compare:

• İkisi de kütüphanede ders çalışıyor.
• Her ikisi de kütüphanede ders çalışıyor.

Both are correct.
In everyday speech, ikisi de is very common and natural.