Vi øver ental og flertal, fordi de samme navneord ofte ser forskellige ud.

In Danish, øve can be used transitively, which means it can take a direct object:

• Vi øver ental og flertal = We practice singular and plural.

You can also meet øve sig or øve sig på, but that is used in other patterns, for example when talking about training yourself or practicing at something in a broader sense.

So in this sentence, øver is simply taking the thing being practiced as its object.

Because ental and flertal are being used as general grammar concepts, not as specific countable items.

Danish often leaves out the article with abstract or category words when speaking generally:

• Vi øver ental og flertal = We practice singular and plural.
• compare English: We study grammar, not usually the grammar in a general statement.

If you were talking about a specific singular form or plural form in context, an article might appear, but here it is a broad statement.

Fordi means because and introduces a reason:

• Vi øver ental og flertal = main idea
• fordi ... = reason for it

So the structure is:

• We practice singular and plural
• because the same nouns often look different

It is a very common conjunction in Danish.

This is a very common Danish pattern.

When a noun is definite and has a word in front of it such as an adjective or a determiner-like word, Danish often uses:

• de
  • adjective/determiner + noun

So:

• navneordene = the nouns
• de samme navneord = the same nouns

You do not usually combine de samme with the definite ending on the noun. That is why it is de samme navneord, not de samme navneordene.

Because samme is one of those words that usually stays the same. It does not inflect the way many ordinary adjectives do.

For example:

• det samme ord = the same word
• de samme ord = the same words
• den samme bog = the same book

So even though Danish adjectives often change form, samme is much more fixed.

Some Danish nouns have the same form in singular and plural indefinite.

That is the case with et navneord / flere navneord.

So:

• et navneord = a noun
• navneord = nouns
• navneordet = the noun
• navneordene = the nouns

This is actually a nice example of what the sentence is talking about: Danish plural forms are not always predictable in the same way as English plural -s.

Because fordi introduces a subordinate clause, and subordinate clauses in Danish usually have different word order from main clauses.

In a main clause, you would more naturally get:

• De samme navneord ser ofte forskellige ud.

But after fordi, the adverb ofte commonly comes before the finite verb:

• ... fordi de samme navneord ofte ser forskellige ud.

This is a very important Danish pattern:

• main clause: verb earlier
• subordinate clause: adverb often before the finite verb

Because se ... ud means look / appear / seem in appearance.

Here the sentence is talking about how the noun forms look on the page. So:

• er forskellige = are different
• ser forskellige ud = look different

The version with ser ... ud puts more focus on appearance or form, which fits well when talking about singular and plural spellings.

Because se ud is a particle verb. In Danish, these are often split in normal sentence patterns.

So the basic verb is:

• at se ud = to look / to appear

But in a sentence, the finite verb and the particle often separate:

• De ser forskellige ud.
• Navneordene så mærkelige ud.

This is similar to separable verbs in some other Germanic languages.

Because the adjective agrees with a plural subject.

The forms are:

• forskellig = common gender singular
• forskelligt = neuter singular
• forskellige = plural or definite

Here the thing being described is de samme navneord, which is plural, so the adjective becomes forskellige.

Compare:

• Et navneord ser forskelligt ud. = A noun looks different.
• Navneord ser forskellige ud. = Nouns look different.

Yes, navneord is a very common school/teaching term for noun. You may also see substantiv, which is more technical or more like traditional grammar terminology.

So:

• navneord = common in language learning and school
• substantiv = more formal grammatical term

Both are correct, but navneord is very natural in a teaching sentence like this one.

The tricky part is ø, which has no exact English equivalent.

A rough guide:

• ø is said with rounded lips
• it is somewhat similar to the vowel in French peu or German schön, though not exactly the same

So øver is roughly:

• Ø-ver

The v is soft, and the whole word is smoother than an English spelling might suggest.

If you are learning pronunciation, ø is worth practicing separately, because it appears in many common Danish words.