De vlieg is klein.

De is the Dutch definite article, usually translated as the.

Dutch nouns take either de or het in the singular. Vlieg is a de-word, so you say de vlieg.

A useful thing to remember:

• de vlieg = the fly
• een vlieg = a fly

Unfortunately, there is no perfect rule that lets you always predict de vs het, so learners usually memorize the article with the noun.

Here, vlieg is a noun, meaning fly (the insect).

A native English speaker might notice that vlieg can also look like a verb form, because:

• ik vlieg = I fly

So how do you know which it is here? The article de gives it away:

• de vlieg = noun phrase, the fly

So in this sentence, it definitely means the insect, not the action of flying.

Because klein comes after the verb is and is being used as a predicate adjective.

In Dutch:

• de kleine vlieg = the small fly
  adjective before the noun → usually takes -e
• de vlieg is klein = the fly is small
  adjective after is → usually no -e

So:

• kleine = attributive adjective
• klein = predicative adjective

This is a very common pattern in Dutch.

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

So:

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

In De vlieg is klein, is works just like English is in The fly is small.

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

• subject: De vlieg
• verb: is
• complement/adjective: klein

So Dutch is behaving very similarly to English here:

• The fly / is / small
• De vlieg / is / klein

More generally, Dutch main clauses usually follow the verb-second rule, meaning the finite verb comes in the second position. In this sentence, the subject is first, so the verb naturally comes right after it.

A rough pronunciation guide:

• de ≈ duh (often unstressed)
• vlieg ≈ vleekh / vleeg
  
  • vl- as in vlog without the o
  • ie sounds like ee in see
  • the final g is usually a throaty Dutch sound in the Netherlands; in some Belgian accents it is softer
• is = like English iss
• klein ≈ kline

A couple of important sound notes:

• ij/ei in klein is often pronounced something like the vowel in eye
• the Dutch g is usually not like the English g in go

So the whole sentence is roughly: duh vleekh iss kline

That is only an approximation, but it is a useful start.

De is capitalized only because it is the first word of the sentence.

Unlike German, Dutch does not capitalize all nouns. So normally you would write:

• de vlieg
• een kleine vlieg

Only the first word of a sentence and proper names are capitalized.

The plural is:

De vliegen zijn klein.

Changes:

• vlieg → vliegen = fly → flies
• is → zijn = is → are
• klein stays klein

So:

• De vlieg is klein. = The fly is small.
• De vliegen zijn klein. = The flies are small.

Also note that all plural nouns in Dutch take de, not het.

Yes. Een vlieg is klein is grammatically correct and means A fly is small or One fly is small, depending on context.

The difference is the article:

• de vlieg = the fly (a specific fly)
• een vlieg = a fly (not specific)

So the sentence structure stays the same; only the article changes.