Die Katze frisst langsam.

Because Katze is a feminine noun in German, so in the nominative singular it takes the article die.

• der = masculine
• die = feminine
• das = neuter

Here, die Katze is the subject of the sentence, so nominative is required.

In German, all nouns are capitalized, not just proper names.

So:

• Katze = noun, so it must be capitalized
• langsam = not a noun, so it stays lowercase

This is one of the most noticeable spelling differences from English.

German often uses fressen for animals eating, while essen is the usual verb for people eating.

So:

• Die Katze frisst. = The cat eats.
• Ich esse. = I eat.

This is a very common distinction in German.
However, in some contexts people may say essen for animals too, especially if they want to sound affectionate or less blunt. But fressen is the standard choice for animals.

frisst is the 3rd person singular present tense form of fressen.

The conjugation is:

• ich fresse
• du frisst
• er / sie / es frisst
• wir fressen
• ihr fresst
• sie / Sie fressen

Notice that the stem vowel changes from e to i in the singular forms du frisst and er/sie/es frisst.

Since die Katze is grammatically she (sie) and singular, frisst is the correct form.

In a normal German main clause, the finite verb usually goes in second position. This is often called the V2 rule.

In this sentence:

• Die Katze = first element
• frisst = second element
• langsam = the rest

So the structure is perfectly standard.

German is not exactly Subject-Verb-Object in the same fixed way as English; what matters more is that the conjugated verb is in position 2 in main clauses.

Because langsam is being used as an adverb here, not an adjective before a noun.

Compare:

• die langsame Katze = the slow cat
  
  • here langsame describes the noun Katze, so it takes an ending
• Die Katze frisst langsam.
  
  • here langsam describes how the cat eats, so it stays unchanged

In German, adverbs usually have the same basic form as the adjective.

Yes. In German, the same word often serves as both the adjective and the adverb.

So:

• eine langsame Katze = a slow cat
• Die Katze frisst langsam. = The cat eats slowly

English usually changes slow to slowly, but German usually does not make that kind of change.

Yes, you might hear that, and it would be understood. But Die Katze frisst langsam is the more typical and standard choice for an animal.

The difference is mainly about usage:

• fressen = standard for animals eating
• essen = standard for humans eating

If someone says Die Katze isst, it may sound a bit more human-like, affectionate, or simply less strict about the distinction.

It is in the present tense, but like the English present tense, it can cover more than one idea depending on context.

It can mean:

• the cat is eating slowly right now
• the cat generally eats slowly
• the cat usually eats slowly

German present tense is very flexible, so context tells you which meaning is intended.

Yes, German word order is somewhat flexible, as long as the finite verb stays in second position in a main clause.

For example:

• Die Katze frisst langsam. = neutral
• Langsam frisst die Katze. = puts emphasis on slowly
• Die Katze langsam frisst = not correct as a normal main clause

So you can move elements for emphasis, but the verb still has to stay in the second position.

frisst is pronounced approximately like frist in English, but with a short, crisp German i sound.

A rough guide:

• fr = similar to English fr
• i = short vowel, like i in sit
• sst = pronounced clearly

So the full sentence sounds roughly like:

Dee KAT-tseh frist LANG-zahm

A few notes:

• die sounds like dee
• Katze has two syllables: KAT-tseh
• langsam is usually LANG-zahm

This is only an approximation, but it is a useful starting point.