De vlieg zit op de tafel.

Because vlieg is a de-word in Dutch. Nouns in Dutch take either de or het, and you usually have to learn this with each noun.

• de vlieg = the fly
• plural: de vliegen = the flies

There is no separate rule here that would let you reliably guess het instead.

For the same reason: tafel is also a de-word.

• de tafel = the table
• plural: de tafels

So both nouns in the sentence happen to take de.

Dutch often uses posture or position verbs where English just uses is. In this sentence, zit comes from zitten, which literally means to sit, but it is also commonly used for something or an animal being positioned somewhere.

So:

• De vlieg is op de tafel would sound less natural in ordinary Dutch.
• De vlieg zit op de tafel is the normal way to say that the fly is on the table.

This does not mean the fly is literally sitting like a person. It just means it is there, resting on the surface.

Here, zit is the third-person singular form of zitten.

Forms of zitten in the present tense:

• ik zit
• jij zit
• hij/zij/het zit
• wij zitten
• jullie zitten
• zij zitten

In this sentence, the subject is de vlieg, which is singular, so the verb is zit.

Yes. They are closely related.

• de vlieg = the fly
• vliegen = to fly

This is one of those useful Dutch word families where the noun and verb look very similar. Just be careful not to confuse:

• de vlieg = the insect
• ik vlieg = I fly

This is the normal Dutch main-clause word order:

subject + verb + rest

So:

• De vlieg = subject
• zit = verb
• op de tafel = prepositional phrase showing location

It works very much like basic English word order here.

You could also say:

• Op de tafel zit de vlieg

That version is also correct, but it gives more emphasis to the location. In Dutch main clauses, the conjugated verb still stays in the second position.

Op means on and is the normal preposition for something being on top of a surface.

So:

• op de tafel = on the table

That is exactly the preposition you would expect here, because the fly is resting on the table’s surface.

Yes, you often can. Both are possible, but there is a small difference in feel.

• op de tafel = on the table, often a specific table
• op tafel = on the table, often more general or idiomatic

In many everyday sentences, Dutch drops the article in expressions like op tafel, in bed, naar school, and so on. But op de tafel is perfectly correct if you mean a particular table.

A rough guide is:

• vlieg sounds somewhat like fleekh, but with the Dutch g at the end

A few useful pronunciation notes:

• vl at the start is pronounced together, as in v-l
• ie sounds like the vowel in English see
• the final g is the harsh Dutch g/ch sound in many accents

So the word is basically vlee + Dutch g.

The plural is vliegen.

• de vlieg = the fly
• de vliegen = the flies

Example:

• De vliegen zitten op de tafel. = The flies are on the table.

Notice that the verb also changes:

• singular: zit
• plural: zitten

Yes, very often. This is an important feature of Dutch.

Dutch uses verbs like:

• zitten = sit / be situated
• staan = stand
• liggen = lie

These are often used for the position of objects, animals, and even abstract things, where English would simply use is or are.

So De vlieg zit op de tafel is a very natural Dutch sentence, even though English would not normally say The fly sits on the table unless you wanted a special stylistic effect.