De leeuw is groot.

Because leeuw is a de-word in Dutch. Dutch nouns take either de or het as their definite article, and you usually have to learn this with each noun.

• de leeuw = the lion
• not het leeuw

A useful thing to know: most singular nouns referring to people or animals are de-words.

Not in a way that is obvious from the modern forms. Leeuw is the normal Dutch word for lion. It does not look similar to the English word, so it is best to learn it as its own vocabulary item.

Plural:

• de leeuw = the lion
• de leeuwen = the lions

This is the normal Dutch word order for a simple main clause:

subject + verb + complement

So here:

• De leeuw = subject
• is = verb
• groot = complement/adjective

It works very much like English The lion is big.

Is is the third-person singular present form of the verb zijn, which means to be.

So:

• ik ben = I am
• jij bent = you are
• hij/zij/het is = he/she/it is

In this sentence, de leeuw is groot means the lion is big.

Because groot comes after the verb is and is being used predicatively. In Dutch, adjectives used this way usually do not take -e.

Compare:

• De leeuw is groot. = The lion is big.
  → predicative adjective, so groot

• De grote leeuw = The big lion
  → adjective directly before the noun, so grote

This is a very common pattern in Dutch.

A rough English approximation is something like lay-oo, but said smoothly and quickly as one Dutch word.

A few helpful points:

• ee is a long vowel.
• uw at the end can be tricky for English speakers.
• The whole word is pronounced as one syllable in careful Dutch pronunciation, not as two clearly separate English syllables.

If you are just starting out, an approximation like layoo will usually be understood, though it is not perfect.

Groot usually means big, large, or sometimes great depending on context.

In De leeuw is groot, the most natural meaning is big/large.

Examples:

• een groot huis = a big house
• een grote fout = a big mistake
• een groot kunstenaar = a great artist

So the exact English translation depends on context, but here big is the straightforward meaning.

Yes. That would mean A lion is big.

The difference is:

• De leeuw is groot.
  Usually the lion is big — often a specific lion, or sometimes a general/class statement depending on context.

• Een leeuw is groot.
  A lion is big — often more general, like talking about lions as a kind of animal.

So both are grammatical, but de and een do not mean the same thing.

You invert the subject and the verb:

• De leeuw is groot. = The lion is big.
• Is de leeuw groot? = Is the lion big?

This is a very common Dutch pattern in yes/no questions: verb + subject + rest

The plural is:

De leeuwen zijn groot.

Changes:

• leeuw → leeuwen = lions
• is → zijn = are

So:

• De leeuw is groot. = The lion is big.
• De leeuwen zijn groot. = The lions are big.

No. De does not mean the animal is male.

In Dutch, de is just the article used with this noun. It is a grammatical feature of the word, not a sign of biological sex.

If you specifically want to say lioness, Dutch uses leeuwin:

• de leeuw = the lion
• de leeuwin = the lioness

Both are de-words.

Normally, only the first word of the sentence is capitalized:

• De leeuw is groot.

Dutch does not capitalize ordinary nouns the way German does. So:

• leeuw stays lowercase
• groot stays lowercase

That is standard Dutch spelling.