Learn Hardcore German: Ich lege meinen Schlüsselbund immer in die obere Schublade. - I always put my key ring in the top drawer.

Because legen is a transitive verb that takes a direct object. In German, direct objects trigger the accusative case. Here Schlüsselbund is masculine singular, so mein (“my”) changes to meinen in the accusative.

How do you decide whether to use accusative or dative after the preposition in?

German prepositions like in can govern either case depending on the sense of motion versus location:

Use accusative when expressing movement into something (direction): “in die Schublade” = into the drawer.
Use dative when expressing a static location inside something: “in der Schublade” = in the drawer (no movement).

Why does obere have an -e ending in obere Schublade?

Because after the definite article die, adjectives take the weak inflection. For a feminine noun in the accusative, the weak ending is -e. The pattern here is:
die (fem. acc.) + ober (stem) + -e → die obere Schublade.

What’s the difference between die obere Schublade and die oberste Schublade?

die obere Schublade simply means “the upper drawer” (one that is above another).
die oberste Schublade uses the superlative oberst-, so it means “the topmost drawer” (the very highest of all).

You use oberste when you want to stress it’s the number-one drawer at the top.

Why is the verb legen used here instead of stellen or setzen?

German distinguishes how you place things:

legen = to lay something down horizontally
stellen = to place something upright
setzen = to set/place people or animals (or sometimes objects)
Since a key ring lies flat, you “lay” it: ich lege … in die Schublade.

Where should the adverb immer be placed in a German sentence, and why is it after the object here?

In a main clause German word order is roughly:
Subject – Verb – (Time) – (Manner) – (Place) – Other Elements.
Here you have:
1) Ich (Subject)
2) lege (Verb)
3) meinen Schlüsselbund (Direct Object)
4) immer (Time adverb)
5) in die obere Schublade (Place phrase)

Time adverbs like immer typically come right after the verb (or after the object if the object is felt to be part of the core message). This placement emphasizes the habitual nature of the action.

Could immer also appear at the beginning of the sentence? What would change?

Yes. You can front a time adverb for emphasis. That gives you:
“Immer lege ich meinen Schlüsselbund in die obere Schublade.”

This inverts subject and verb (V-S order) and stresses “always” even more, perhaps contrasting with times when you didn’t.

Why are Schlüsselbund and Schublade capitalized, while words like in and immer are not?

In German, all nouns are capitalized by rule (e.g. Schlüsselbund, Schublade). Words that are not nouns—prepositions (in) or adverbs (immer)—remain lowercase.

Can we drop the article die before obere Schublade in German?

No. Unlike English (“in top drawer”), German generally requires a determiner (definite, indefinite, possessive) before singular countable nouns. You need die (or eine, meine, etc.).

How would the sentence look if we used the separable verb hineinlegen?

You can say:
“Ich lege meinen Schlüsselbund immer in die obere Schublade hinein.”

Here hinauslegen is “to lay into,” and as a separable verb, its prefix hin- goes to the end of the clause.