Tom viel gisteren op zijn knie, en nu doet ook zijn enkel pijn.

Viel is the simple past of vallen (to fall).

• vallen → infinitive
• viel → simple past, singular
• is gevallen → present perfect

Dutch often uses the simple past for a finished event in a narrative sentence, especially with a time word like gisteren (yesterday).

So:

• Tom viel gisteren... = Tom fell yesterday...
• Tom is gisteren gevallen... would also be possible in everyday Dutch, but viel is very natural here.

Yes. Vallen is irregular.

Its main forms are:

• vallen = to fall
• viel = fell
• gevallen = fallen

So you cannot build the past by just adding a regular ending. It has to be learned as an irregular form.

Because the sentence says he fell on one knee.

• op zijn knie = on his knee
• op zijn knieën = on his knees

Dutch uses the singular here if only one knee is meant. If Tom landed on both knees, then op zijn knieën would make more sense.

Op means on/onto, and here it matches the idea of landing on that body part.

• op zijn knie vallen = to fall on/onto his knee

This is very similar to English. Dutch uses op naturally in this kind of physical contact situation.

Because they already have the possessive zijn (his).

In Dutch, just like in English, you normally do not use an article together with a possessive:

• zijn knie = his knee
• zijn enkel = his ankle

Not:

• de zijn knie
• de zijn enkel

So the possessive replaces the article.

Pijn doen is a very common Dutch expression meaning to hurt or to be painful.

So:

• Zijn enkel doet pijn = His ankle hurts
• literally: His ankle does pain

This sounds strange if translated word for word into English, but it is normal Dutch.

You can also see it in plural:

• Mijn benen doen pijn = My legs hurt

So the verb agrees with what hurts:

• singular: doet pijn
• plural: doen pijn

Normally, no. Dutch usually says iets doet pijn for a body part that hurts.

Natural Dutch:

• Mijn arm doet pijn
• Zijn enkel doet pijn

Less natural or wrong in this structure:

• Zijn enkel heeft pijn

Dutch can use pijn hebben, but usually with a person, not with the body part itself:

• Hij heeft pijn = He is in pain
• Hij heeft pijn aan zijn enkel = He has pain in his ankle

So:

• Zijn enkel doet pijn = His ankle hurts
• Hij heeft pijn aan zijn enkel = He has pain in his ankle

Both are fine, but they are different structures.

This is because Dutch is a verb-second (V2) language in main clauses.

The first position is taken by nu (now), so the finite verb must come next:

1. nu
2. doet
3. the rest of the sentence

So:

• Nu doet ook zijn enkel pijn.

If the subject comes first, you get:

• Ook zijn enkel doet nu pijn.
• Zijn enkel doet nu ook pijn.

These are all possible, but they place the emphasis slightly differently.

Here ook means also/too, and it emphasizes that his ankle hurts as well.

The idea is:

• first, he fell on his knee
• now, his ankle also hurts

So ook connects the new pain in the ankle to the earlier injury.

In this sentence, ook is closely linked to zijn enkel:

• nu doet ook zijn enkel pijn = now his ankle too hurts

Because both body parts belong to Tom:

• zijn knie = his knee
• zijn enkel = his ankle

Dutch often repeats the possessive where English might also repeat it for clarity. It sounds completely natural here.

Yes. Z'n is an informal spoken/written reduction of zijn.

So these are equivalent in meaning:

• op zijn knie
• op z'n knie

The full form zijn is more standard in careful writing. Z'n is very common in everyday language.

Because Dutch main clauses typically put the finite verb early in the sentence.

Here the structure is:

• Tom = subject
• viel = finite verb
• gisteren = time expression
• op zijn knie = prepositional phrase

So:

• Tom viel gisteren op zijn knie

This is a very normal Dutch order. Other orders are possible for emphasis, such as:

• Gisteren viel Tom op zijn knie

But the original version is perfectly natural.

Here enkel is a noun meaning ankle.

Dutch enkel can also be an adjective/adverb meaning only/single in other contexts, but not here.

In this sentence:

• zijn enkel = his ankle

Because it follows the possessive zijn and functions as a body-part noun, the meaning is clearly ankle.

Yes, absolutely. That is another natural way to express the same idea.

Compare:

• Nu doet ook zijn enkel pijn = Now his ankle hurts too
• Nu heeft hij ook pijn aan zijn enkel = Now he also has pain in his ankle

The first focuses on the body part itself as the thing that hurts.
The second focuses more on Tom as the person experiencing pain.

Both are correct and common.