De baby ligt op de buik.

Ligt is from liggen, which means to lie or to be lying.

In Dutch, it is very common to use a position verb instead of a general verb like to be. For someone or something in a horizontal position, Dutch usually uses liggen.

So:

• De baby ligt op de buik. = The baby is lying on its stomach/tummy.

A sentence with is would sound much less natural here.

Ligt is the third-person singular present tense of liggen.

The present tense forms are:

• ik lig = I lie / am lying
• jij ligt = you lie / are lying
• hij/zij/het ligt = he/she/it lies / is lying
• wij/jullie/zij liggen = we/you/they lie / are lying

Since de baby is a singular subject, Dutch uses ligt.

They are related, but they are not the same.

• liggen = to lie / to be lying
  
  • no direct object
  • describes a position
• leggen = to lay / to put down
  
  • takes a direct object
  • describes causing something to be in that position

Compare:

• De baby ligt op de buik. = The baby is lying on its stomach.
• Ik leg de baby op de buik. = I lay/put the baby on its stomach.

This is similar to the English lie vs lay distinction.

Dutch often uses the definite article with body parts when it is already obvious whose body part is meant.

So op de buik can naturally mean:

• on its stomach
• on his stomach
• on her stomach

In English, we usually prefer a possessive: on his/her/its stomach. Dutch often does not need that.

So these are both possible:

• De baby ligt op de buik.
• De baby ligt op zijn buik.

The version with de buik sounds more general and idiomatic. The version with zijn buik is more explicit.

Because baby is a de-word in standard Dutch:

• de baby

This is something you usually just have to learn with the noun.

A useful comparison:

• de baby = the baby
• het kindje = the little child

So even though a baby is young and small, the noun baby itself still takes de.

It can match several English words depending on context.

Buik literally refers to the belly/abdomen area. In natural English translation, this sentence is often rendered as:

• The baby is lying on its stomach.
• The baby is lying on its tummy.

For babies, tummy is often the most natural English choice. So the exact English word depends on style, not on a different Dutch grammar point.

Yes, very much so. Dutch commonly uses expressions like:

• op de buik liggen = to lie on one’s stomach
• op de rug liggen = to lie on one’s back
• op de zij liggen = to lie on one’s side

So op de buik is a very normal, established way to describe body position.

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

• De baby = subject
• ligt = finite verb
• op de buik = prepositional phrase

So the pattern is:

subject + verb + rest

Dutch main clauses normally put the finite verb in the second position, and that is exactly what happens here.

You can change the emphasis by moving another element to the front:

• Op de buik ligt de baby.

That is grammatically possible, but the neutral version is:

• De baby ligt op de buik.

Buik is pronounced roughly like b-owk, but the vowel ui is a special Dutch sound that does not exist exactly in English.

A rough guide:

• b as in English bed
• ui = a difficult Dutch vowel, somewhat between ow and uh
• k as in cat

The hardest part is ui. That same sound appears in words like:

• huis = house
• muis = mouse

So if you struggle with buik, that is completely normal for English speakers.