Ik leg de sleutel in de tas en stop het bonnetje erin.

Dutch nouns take either de or het.

• sleutel is a de-word, so you say de sleutel
• bonnetje is a het-word, so you say het bonnetje

A very useful rule here is that all diminutives in Dutch are het-words. Since bonnetje is a diminutive, it automatically takes het.

So even if you are not sure about the base noun, het bonnetje is correct because of the ending.

The ending -tje is a diminutive ending.

So:

• bon = receipt, ticket, voucher, coupon, depending on context
• bonnetje = little receipt / receipt slip

In real Dutch, diminutives are used very often, and they do not always mean something is literally small. Sometimes they just sound more natural or everyday.

In this sentence, bonnetje is a very normal way to say receipt.

Because Dutch usually uses in for both:

• in = in
• in = into

The idea of movement is usually shown by the verb, not by changing in to a different word.

So:

• Ik leg de sleutel in de tas = I put/lay the key into the bag
• De sleutel ligt in de tas = The key is in the bag

Dutch uses in in both cases, but the verb tells you whether it is movement or location.

Erin means in it or in there.

It is made from:

• er
  • in = erin

Dutch very often uses these er-words to refer back to a thing or place that was just mentioned.

So after de tas has already been mentioned, Dutch prefers:

• Ik leg de sleutel in de tas en stop het bonnetje erin

instead of repeating:

• Ik leg de sleutel in de tas en stop het bonnetje in de tas

Both are grammatical, but erin sounds more natural because it avoids repetition.

Yes, absolutely.

Both are correct:

• Ik leg de sleutel in de tas en stop het bonnetje erin
• Ik leg de sleutel in de tas en ik stop het bonnetje erin

In the original sentence, the second ik is omitted because the subject is still the same. Dutch often does this with en when one person does both actions.

So the version without the second ik is just a smoother, less repetitive way to say it.

Because the subject is ik.

In the present tense, ik usually takes the bare stem of the verb:

• leggen → ik leg
• stoppen → ik stop

Compare:

• ik leg
• jij legt
• hij legt

and

• ik stop
• jij stopt
• hij stopt

So leg and stop are exactly what you expect with ik.

The two verbs have slightly different feels.

• leggen = to lay / place something down
• stoppen = to put / stick / stuff something into something

So:

• de sleutel is being placed in the bag
• het bonnetje is being put/stuck in there

This sounds natural because a receipt is a thin piece of paper, and stoppen works well for putting something into a bag, pocket, envelope, etc.

That said, Dutch also often uses doen as a general verb meaning put in everyday speech:

• Ik doe de sleutel in de tas

So the original sentence is natural, but it is not the only possible wording.

In a simple main clause like this, erin often comes after the direct object:

• Ik stop het bonnetje erin

That is a very normal word order.

If you use the full noun instead, you also get:

• Ik stop het bonnetje in de tas

So a good pattern to remember is:

• verb + object + place
• stop + het bonnetje + erin

Yes, it is grammatically correct.

It just sounds more repetitive because in de tas is repeated. Dutch usually prefers erin once the place has already been introduced.

So:

• ... in de tas ... erin = more natural
• ... in de tas ... in de tas = correct, but repetitive

That is why the original sentence sounds smoother to a Dutch speaker.