Learn Hardcore German: Ich stecke das Wechselgeld in meine Geldbörse.

Because ich requires the 1st person singular present: ich stecke. Quick paradigm: ich stecke, du steckst, er/sie/es steckt, wir stecken, ihr steckt, sie/Sie stecken. In casual speech you may hear ich steck (dropping the final -e).

What case does in take here?

in is a two-way preposition. With motion into something (answering Wohin?), it takes accusative: in meine Geldbörse. With static location (answering Wo?), it takes dative: in meiner Geldbörse.

Why meine Geldbörse and not meiner Geldbörse?

Because after in with motion you need the accusative. Geldbörse is feminine; the accusative feminine form of the possessive mein- is meine. Dative feminine would be meiner (used for location: in meiner Geldbörse = in my wallet).

Is das Wechselgeld accusative or nominative?

Accusative as the direct object of stecken. For neuter nouns, nominative and accusative both look like das, so the article doesn’t change.

Can I say in meiner Geldbörse in this sentence?

Not if you mean “put into.” In meiner Geldbörse would be dative (location). You’d use it like: Das Wechselgeld ist in meiner Geldbörse (The change is in my wallet). For motion, stick with in meine Geldbörse.

Why use stecken here and not legen or tun?

stecken emphasizes inserting something into an opening/enclosed space (slot, pocket, wallet). Perfect for coins going into a wallet.
legen = to lay/place so it lies flat. It can be used with containers, but the “insert” nuance is weaker: Ich lege das Geld in die Geldbörse is okay.
tun is very generic (“to put/do”) and sounds casual: Ich tue das Wechselgeld in meine Geldbörse. Fine in speech, less precise.

What’s the difference between Wechselgeld and Kleingeld?

Wechselgeld = the change you get back after paying (the returned amount).
Kleingeld = small change/coins you have on you. Regionally, Rückgeld also means the returned change.

Are there other common words for Geldbörse?

Yes, and they vary by region/register:

das Portemonnaie / Portmonee (very common; CH especially uses Portemonnaie)
der Geldbeutel (southern Germany)
die Brieftasche (often a larger wallet, esp. for men)
Austria: das Börsel, die Geldtasche Watch the article/gender.

Can I move the parts around? What about Das Wechselgeld stecke ich in meine Geldbörse?

German allows flexible word order for emphasis. All are correct:

Neutral: Ich stecke das Wechselgeld in meine Geldbörse.
Emphasis on object: Das Wechselgeld stecke ich in meine Geldbörse.
Emphasis on destination: In meine Geldbörse stecke ich das Wechselgeld. Keep the finite verb in second position in main clauses.

How do I pronounce tricky parts like Wechsel- and ö in Geldbörse?

Wechsel-: chs is pronounced like ks (roughly VEK-sel).
ö in Börse is a rounded vowel; say the vowel of English “bird” (in many accents) but round your lips. A safe trick: say “eh” and round your lips.
Final -se in Börse sounds like -ze [zə], not like an unvoiced s.

What would the same idea look like in the past?

Simple past (Präteritum): Ich steckte das Wechselgeld in meine Geldbörse.
Present perfect (spoken past): Ich habe das Wechselgeld in meine Geldbörse gesteckt.

When can I use ins?

ins = in das (accusative, neuter). You can use it with neuter destinations: Ich stecke das Wechselgeld ins Portemonnaie. You cannot say ins meine Geldbörse; with a possessive you keep in meine.

Can I drop the article before Wechselgeld?

Yes, but it changes the nuance.

Ich stecke das Wechselgeld ... = the specific change (e.g., what the cashier just gave you).
Ich stecke Wechselgeld ... = some change (non-specific). Both are grammatical.

What gender and plural does Geldbörse have, and why?

Geldbörse is feminine (die Geldbörse), plural die Geldbörsen. In German compounds, the last element determines the gender; here the head is Börse (feminine).

How would I replace the nouns with pronouns naturally?

For Wechselgeld (neuter): Ich stecke es in meine Geldbörse.
To avoid repeating the place, use particles:
- Motion into: hinein/rein → Ich stecke es hinein / ich steck es rein.
- Static location: darin/drin → Das Wechselgeld ist darin/drin. Using in sie for a thing is possible but less idiomatic than hinein/rein or da rein.