استلفت كتاب من زميلتي، ووعدتها ارجعه بكرا.

I borrowed a book from my female colleague, and I promised her I would return it tomorrow.

Breakdown of استلفت كتاب من زميلتي، ووعدتها ارجعه بكرا.

• ي: my
• من: from
• كتاب: book
• و: and

Questions & Answers about استلفت كتاب من زميلتي، ووعدتها ارجعه بكرا.

Is this sentence Levantine Arabic or Modern Standard Arabic?

It is Levantine colloquial Arabic.

A few clues:

• بكرا is the everyday Levantine word for tomorrow
• there are no case endings
• the structure is more conversational than formal written Arabic

A more MSA-like version would be:

استعرتُ كتابًا من زميلتي، ووعدتُها أن أُعيده غدًا.

So the sentence you have is natural for spoken Levantine, not formal Standard Arabic.

Why does استلفت mean I borrowed here?

In Levantine, استلف commonly means to borrow.

So:

• استلفت كتاب = I borrowed a book

A related verb is:

• سلّف = to lend

So:

• سلّفني كتاب = he/she lent me a book

This is one place where dialect and MSA differ a bit, because in MSA learners often meet استعار for borrow.

Why is there no separate word for I in the sentence?

Because the verb already shows the subject.

In استلفت and وعدت:

• the ending -ت tells you the subject is I

So:

• استلفت = I borrowed
• وعدت = I promised

Arabic often leaves out subject pronouns when the verb already makes them clear.