Am Sonntag nehme ich ein heißes Bad in der Badewanne, um mich zu entspannen.

Am Sonntag means on Sunday.

• am = an dem
• With days of the week, German normally uses am:
  • am Montag = on Monday
  • am Freitag = on Friday
  • am Sonntag = on Sunday

So Am Sonntag is the standard way to say on Sunday.

German main clauses usually follow the verb-second rule.

That means:

• The finite verb must be in the second position
• If something other than the subject comes first, the subject moves after the verb

So:

• Ich nehme am Sonntag ein heißes Bad.
• Am Sonntag nehme ich ein heißes Bad.

Both are correct. In the second version, Am Sonntag is placed first for emphasis, so the verb nehme comes next, and then ich.

This is simply a common German expression.

• ein Bad nehmen = to take a bath
• baden = to bathe / to go bathing

In everyday German, ein Bad nehmen is very natural when talking about taking a bath in a bathtub.

Examples:

• Ich nehme ein Bad. = I’m taking a bath.
• Sie nimmt ein heißes Bad. = She is taking a hot bath.

So this works a lot like English take a bath, even though the literal verb is nehmen = take.

Because Bad is a neuter noun: das Bad.

Here it is the direct object after nehmen, so it is in the accusative case.

For a neuter noun in the accusative with ein, the adjective ending is -es:

• ein heißes Bad

Quick breakdown:

• ein = a
• heißes = hot
• Bad = bath

Compare:

• ein heißer Tee (masculine)
• eine heiße Suppe (feminine)
• ein heißes Bad (neuter)

They are related, but they do not mean the same thing.

• das Bad = bath
  This can mean the bath itself, or the act of bathing in this sentence.
• die Badewanne = bathtub

So:

• ein heißes Bad nehmen = to take a hot bath
• in der Badewanne = in the bathtub

The sentence uses both because one refers to the activity, and the other refers to the place.

Because here in describes location, not movement.

German two-way prepositions like in can take:

• dative for location: where?
• accusative for direction: where to?

In this sentence:

• in der Badewanne = in the bathtub
  This answers where?, so it uses the dative

Examples:

• Ich liege in der Badewanne. = I’m lying in the bathtub.
• Ich steige in die Badewanne. = I’m getting into the bathtub.

So in der Badewanne is correct because the sentence describes where the bath happens.

Because sich entspannen is commonly used as a reflexive verb in German.

• sich entspannen = to relax
• mich is the reflexive pronoun for ich

So:

• ich entspanne mich = I relax
• um mich zu entspannen = in order to relax

This is different from English, where we usually just say to relax without a reflexive pronoun.

Reflexive forms:

• ich entspanne mich
• du entspannst dich
• er/sie entspannt sich

um ... zu means in order to or so as to.

It introduces a purpose clause: it explains why the person is taking the bath.

So:

• um mich zu entspannen = in order to relax

Structure:

• um
  • rest of clause + zu
    • infinitive at the end

Examples:

• Ich lerne Deutsch, um in Deutschland zu arbeiten.
• Sie geht spazieren, um sich zu beruhigen.

In your sentence, the purpose is:

• taking a hot bath
• in order to relax

Because in an um ... zu clause, the infinitive goes at the end.

This is a fixed German pattern:

• um
  • other elements + zu + infinitive

So:

• um mich zu entspannen

Not:

• um zu mich entspannen
• um entspannen zu mich

The normal word order is:

1. um
2. other words, if any
3. zu + infinitive at the end

It is grammatically correct, but in der Badewanne can sound a bit redundant, because a bath normally already happens in a bathtub.

So German speakers might often simply say:

• Am Sonntag nehme ich ein heißes Bad, um mich zu entspannen.

Adding in der Badewanne is still possible if you want to be extra explicit, contrast it with something else, or emphasize the location.

So:

• with it = correct, slightly more explicit
• without it = often more natural in everyday speech

It can sometimes depend on context.

• Am Sonntag often means on Sunday / this Sunday
• If the sentence describes a routine, it can also be understood more generally

However, if you clearly mean every Sunday, German often says:

• Jeden Sonntag nehme ich ein heißes Bad.

That is the clearest way to express a repeated habit:

• jeden Sonntag = every Sunday

So:

• Am Sonntag = on Sunday / this Sunday
• Jeden Sonntag = every Sunday