Learn Hardcore German: Ich erhitze die Suppe im Topf.

im is the contraction of in dem (dative, masculine/neuter singular). So im Topf = in dem Topf. With feminine nouns you don’t contract: you say in der Pfanne, not “im Pfanne”.

Why is it die Suppe and not der Suppe?

Because die Suppe is the direct object (accusative). Feminine nouns have the same form in nominative and accusative: die. der Suppe would be dative or genitive (e.g., “mit der Suppe” = with the soup).

Why is it dative in im Topf?

in is a two-way preposition:

Dative for location (where something is): im Topf = in the pot.
Accusative for motion/direction (where something goes): in den Topf = into the pot.

Can I say in den Topf instead?

Yes, but only if you mean movement into the pot: e.g., Ich gieße die Suppe in den Topf. In your sentence we’re talking about where the soup is being heated (location), so dative: im Topf.

Is erhitzen separable? How do I form the past?

erhitzen is inseparable (prefix er- never separates).

Perfekt: Ich habe die Suppe erhitzt. (no “ge-” added)
Präteritum: Ich erhitzte die Suppe.

Which verb should I use: erhitzen, aufwärmen, erwärmen, kochen, or heizen?

erhitzen: to heat up (often to a higher temperature; neutral/technical). Works well here.
aufwärmen: to reheat something already cooked (leftovers).
erwärmen: to warm (moderate increase; a bit formal/literary; also figurative).
kochen: to cook; also “to boil.” Ich koche die Suppe = I’m making the soup.
heizen: to heat a space (a room/heating system), not food.

How do I conjugate erhitzen in the present?

ich erhitze
du erhitzst (colloquially often pronounced like “erhitzst”; standard spelling is erhitzt for du? Note: correct forms are: du erhitzt, er/sie/es erhitzt; Keep this straight.) Wait, correct present tense:
ich erhitze
du erhitzt
er/sie/es erhitzt
wir erhitzen
ihr erhitzt
sie/Sie erhitzen

How do you pronounce the tricky parts?

Ich: the “ch” is the soft German sound [ç] (not like English “k”).
erhitze: the z sounds like “ts” (er-HIT-tsuh); final -e is a schwa.
Suppe: short “u” (like in “put”), doubled consonant marks the short vowel.
Topf: pronounce the pf cluster; short “o.”

Can I change the word order?

Yes, as long as the finite verb stays in second position in a main clause:

Im Topf erhitze ich die Suppe. (emphasis on location)
Die Suppe erhitze ich im Topf. (emphasis on the soup) In a subordinate clause, the verb goes to the end: …, weil ich die Suppe im Topf erhitze.

Should it be in einem Topf instead of im Topf?

Use in einem Topf if the pot is non-specific. im Topf (= in dem Topf) implies a specific, known pot from context. Both are grammatical; the choice is about definiteness.

Can I drop ich like in Spanish?

No. German generally requires the subject pronoun. To give a command, use the imperative:

Singular informal: Erhitze die Suppe im Topf!
Plural informal: Erhitzt die Suppe im Topf!
Formal: Erhitzen Sie die Suppe im Topf!

Why are Suppe and Topf capitalized?

All nouns are capitalized in German. Suppe and Topf are nouns, so they take capitals.

Why not ins Topf?

ins = in das (accusative neuter). Topf is masculine (der Topf), so the accusative is in den Topf, not “ins Topf.” Example where ins works: ins Glas (das Glas).

Is Ich erhitze Suppe OK (without an article)?

It can be, in “recipe style” or when talking about an indefinite amount: Suppe erhitzen und servieren. In everyday speech you typically use an article: Ich erhitze die Suppe (specific) or Ich erhitze etwas Suppe (some soup).

How do I add information like “on the stove” or time expressions?

Add more adverbials, keeping verb-second:

Place: Ich erhitze die Suppe im Topf auf dem Herd.
Time: Ich erhitze die Suppe heute Abend im Topf. German often orders adverbials roughly as time–manner–place, but it’s flexible for emphasis.