Ich stecke den Kassenbon in die Jackentasche.

Stecken is the infinitive, meaning to put/stick. In the sentence, the verb has to match the subject ich, so it becomes ich stecke.

Present tense of stecken:

• ich stecke
• du steckst
• er/sie/es steckt
• wir stecken
• ihr steckt
• sie/Sie stecken

Because den Kassenbon is the direct object of the verb: it is the thing being put somewhere.

The basic noun is der Kassenbon. Since it is masculine, its article changes in the accusative:

• nominative: der Kassenbon
• accusative: den Kassenbon

So:

• Ich stecke den Kassenbon ...

Because in is a two-way preposition. It can take:

• accusative for direction / movement into
• dative for location / being inside

Here, the receipt is being moved into the pocket, so German uses the accusative:

• in die Jackentasche = into the jacket pocket

Compare:

• Ich stecke den Kassenbon in die Jackentasche. = movement into it
• Der Kassenbon ist in der Jackentasche. = it is inside it

Because Jackentasche is a feminine singular noun: die Jackentasche.

In this sentence, it is in the accusative, but for many feminine nouns, the article stays die in both nominative and accusative:

• nominative: die Jackentasche
• accusative: die Jackentasche

So die here does not mean plural. It is feminine singular.

It is a compound noun, which German uses very often.

Jackentasche is made from:

• Jacke = jacket
• Tasche = pocket / bag

So Jackentasche literally means something like jacket-pocket, i.e. jacket pocket.

A very important rule with German compounds is: the last part decides the gender. Since die Tasche is feminine, die Jackentasche is also feminine.

Kassenbon is also a compound noun:

• Kasse = cash register / checkout
• Bon = receipt slip

The final part is Bon, and der Bon is masculine. That is why the whole word is:

• der Kassenbon

And in the accusative:

• den Kassenbon

Not exactly.

Stecken often suggests putting something into a relatively small, enclosed, or tight place, such as:

• a pocket
• a bag
• an envelope
• a socket

So it fits very well here: putting a receipt into a jacket pocket.

Compare:

• legen = to lay/place something down
• tun = to put/do, but often more general or colloquial
• stecken = to stick/put into

So stecken sounds more natural than legen in this sentence.

That is the most neutral word order here.

In a normal German main clause, the finite verb comes second:

• Ich stecke ...

After that, German often places the direct object before a prepositional phrase:

• Ich stecke den Kassenbon in die Jackentasche.

Other orders are possible, but they usually sound more marked or emphasize something:

• In die Jackentasche stecke ich den Kassenbon.

That version puts extra focus on where it is going.

Yes, absolutely.

Adding mir is very common in German. It shows that the pocket is mine or that I am doing this for myself / on my person.

So:

• Ich stecke den Kassenbon in die Jackentasche. = neutral
• Ich stecke mir den Kassenbon in die Jackentasche. = very natural, with the idea into my jacket pocket

German often uses this kind of dative pronoun with clothing and body-related things.

Because German often uses case to show the difference that English shows with separate words like in vs into.

With in:

• accusative = into
• dative = in

So:

• in die Jackentasche = into the jacket pocket
• in der Jackentasche = in the jacket pocket

English uses different wording; German often uses the same preposition but changes the case.

Yes.

• in die Jackentasche = into the jacket pocket
• in meine Jackentasche = into my jacket pocket

Both are correct. The version with meine makes the ownership explicit. The version with die is fine when the context already makes it clear whose pocket it is.

In everyday German:

• Kassenbon = cash-register receipt / store receipt
• Bon = often a shorter everyday way to say the same thing
• Quittung = receipt, but often in a slightly more formal sense, sometimes a written acknowledgment of payment

So in a shopping context, Kassenbon is a very natural word for the slip you get at the checkout.