Dit is die winkelmandjie waarin ons die hemp en die broek sit.

Because Afrikaans normally writes compound nouns as a single word.

• winkel = shop/store
• mandjie = basket

So winkelmandjie literally means shop-basket or shopping basket. This is very common in Afrikaans: instead of writing separate words, you usually join them into one compound noun.

It is a diminutive ending. The base word is mand = basket, and mandjie means little basket.

In Afrikaans, diminutives are extremely common and often sound natural even when English would not use little. So winkelmandjie is simply the normal everyday word for a shopping basket.

waarin means in which.

It is made from:

• waar = where
• in = in

When Afrikaans refers back to a thing after a preposition, it often uses these combined forms:

• waarin = in which
• waarop = on which
• waarmee = with which

So die winkelmandjie waarin ons ... sit means the shopping basket in which we put ...

Because that part is a subordinate clause, specifically a relative clause.

Main clause:

• Dit is die winkelmandjie.

Relative clause:

• waarin ons die hemp en die broek sit

In Afrikaans subordinate clauses, the finite verb usually goes to the end. That is why sit comes after the subject ons and the objects die hemp en die broek.

That is a very common learner question. In Afrikaans, sit can indeed mean sit, but it is also often used to mean put/place, especially in everyday speech.

So here:

• ons sit die hemp en die broek in die winkelmandjie

means

• we put the shirt and the trousers into the shopping basket

This is normal Afrikaans usage.

Because die is the definite article for basically all nouns in standard Afrikaans, regardless of gender.

So you get:

• die winkelmandjie
• die hemp
• die broek

Unlike some other languages, Afrikaans does not have different forms like masculine/feminine/neuter definite articles. die works for all of these.

Because both nouns are definite, and each noun normally keeps its own article:

• die hemp en die broek

This clearly means the shirt and the trousers. Repeating the article is the normal, clear way to say it.

In Afrikaans, broek is grammatically singular, even though English often uses a plural word like trousers or pants.

So:

• die broek = the trousers / the pants

If you mean more than one pair, then the plural is broeke.

This is one of those places where English and Afrikaans do not match neatly.

Yes, that kind of split structure is also possible and very common in speech:

• Dit is die winkelmandjie waar ons die hemp en die broek in sit.

That said, waarin is a very standard and neat form, especially in careful written Afrikaans. So both are useful to recognize, but waarin is an excellent form to learn.