Tafadhali weka vitabu ndani ya begi.

Tafadhali means please. It makes the command more polite.

Without it, Weka vitabu ndani ya begi is still correct, but it sounds more direct: Put the books in the bag.
With tafadhali, it becomes a polite request: Please put the books in the bag.

You can also move tafadhali around a little, for example:

• Tafadhali weka vitabu ndani ya begi.
• Weka vitabu ndani ya begi, tafadhali.

Both are natural.

Yes. Weka is the imperative form of the verb -weka, meaning to put / place.

In Swahili commands, the subject you is usually not said, because it is understood:

• Weka = Put!

So the sentence does not need a separate word for you.

Weka is normally used when speaking to one person.

If you are speaking to more than one person, the plural command is:

• Wekeni vitabu ndani ya begi.

So:

• Weka = put! (to one person)
• Wekeni = put! (to several people)

Vitabu means books. The singular is kitabu, meaning book.

This is part of Swahili’s noun class system:

• kitabu = book
• vitabu = books

This ki-/vi- pairing is very common in Swahili.

Swahili usually does not have separate words for a/an and the the way English does.

So:

• begi can mean a bag or the bag
• vitabu can mean books or the books

Context tells you which meaning is intended.

If you want to be more specific, Swahili often uses demonstratives instead:

• begi hili = this bag
• begi lile = that bag

Ndani ya means inside of or in(side) the.

So:

• ndani ya begi = inside the bag

You can think of it as a fixed expression:

• ndani = inside/interior
• ya = of

Very literally, it is something like the inside of the bag, but in normal English we translate it as in the bag or inside the bag.

In expressions like ndani ya, juu ya, mbele ya, and chini ya, the ya links the relational word to the following noun.

Examples:

• ndani ya nyumba = inside the house
• juu ya meza = on top of the table
• mbele ya shule = in front of the school

For a learner, the easiest way is to memorize ndani ya as one useful chunk meaning inside/in.

Yes, often you can.

• ndani ya begi strongly emphasizes inside the bag
• kwenye begi is a more general locative expression that can often mean in the bag in everyday speech

So:

• Weka vitabu ndani ya begi = Put the books inside the bag
• Weka vitabu kwenye begi = Put the books in the bag

Both can be natural, but ndani ya is a bit more explicit about being inside.

Pretty close, yes.

The sentence follows this order:

• Tafadhali = please
• weka = put
• vitabu = the books
• ndani ya begi = in the bag

So it is basically:

Please + put + books + in the bag

That is very similar to English word order.

Begi is a borrowed word, ultimately from English bag. It is very common in modern Swahili.

Its plural is often:

• begi = bag
• mabegi = bags

Swahili uses many loanwords, especially for everyday objects, and they behave like normal Swahili nouns once borrowed.

This is where noun classes become important.

Since vitabu is plural of kitabu, other words connected to it often change to match its noun class.

For example:

• vitabu hivi = these books
• vitabu vikubwa = big books

And with begi:

• begi hili = this bag
• begi kubwa = big bag

So even in a simple sentence like this one, vitabu and begi belong to different noun classes, and that affects agreement in longer phrases.