Ik voel mijn voorhoofd zweten en mijn arm jeuken zodra ik begin te rennen.

Dutch often uses a perception verb + object + infinitive pattern after verbs like zien (see), horen (hear), and voelen (feel).

So:

• Ik voel mijn voorhoofd zweten = I feel my forehead sweating
• Ik voel mijn arm jeuken = I feel my arm itching

This is very natural in Dutch and is similar to English I feel my arm itch or I feel my forehead sweating.

You can also use a clause with dat, but it usually sounds more explicit or less direct:

• Ik voel dat mijn voorhoofd zweet.

That means something like I can feel that my forehead is sweating, which is grammatical, but the original sentence is more idiomatic here.

Because after voelen in this kind of construction, Dutch uses the bare infinitive.

Pattern:

• ik voel + object + infinitive

Examples:

• Ik voel mijn hart bonzen. = I feel my heart pounding.
• Ik voel mijn benen trillen. = I feel my legs shaking.
• Ik voel mijn arm jeuken. = I feel my arm itching.

So zweten and jeuken are not conjugated here because they belong to the things being perceived:

• mijn voorhoofd is doing the sweating
• mijn arm is doing the itching

Because jeuken here is not the main finite verb of the sentence. The main verb is voel.

Compare:

• Mijn arm jeukt. = My arm itches.
  Here jeukt is conjugated because it is the main verb.

But in the original sentence:

• Ik voel mijn arm jeuken.

The structure is like:

• I feel [my arm itch]

So jeuken stays as an infinitive.

Dutch usually repeats the possessive adjective when two separate body parts are mentioned.

So this is natural:

• mijn voorhoofd en mijn arm

You may sometimes see the possessive only once in other contexts, but with body parts, repeating it is very normal and clear.

For a learner, it is safest to copy this pattern:

• Ik was mijn handen.
• Ik strek mijn rug en mijn nek.
• Ik voel mijn voorhoofd zweten en mijn arm jeuken.

Zodra means as soon as.

In this sentence:

• zodra ik begin te rennen = as soon as I start running

Compared with similar words:

• als = if / when
• wanneer = when
• zodra = as soon as

So zodra is more specific: it emphasizes that the second thing happens immediately after the first.

Examples:

• Ik bel je zodra ik thuis ben. = I’ll call you as soon as I’m home.
• Als ik thuis ben, bel ik je. = When/if I’m home, I’ll call you.

After beginnen, Dutch normally uses te + infinitive.

So:

• beginnen te rennen = to begin to run
• begin te lezen = begin to read
• begint te regenen = begins to rain

That is why the sentence says:

• ik begin te rennen

This is a very common pattern. English learners should remember that Dutch often uses te where English uses to.

In modern Dutch, begin te rennen is the normal and most natural order.

So the subordinate clause is:

• zodra ik begin te rennen

Here:

• zodra introduces a subordinate clause
• ik is the subject
• begin is the conjugated verb
• te rennen stays together as the infinitive phrase

You may encounter other verb orders in formal or older-style Dutch, but for everyday modern Dutch, ik begin te rennen is the standard choice.

Because Dutch allows flexible word order, and the speaker has chosen to focus first on the physical sensation:

• Ik voel mijn voorhoofd zweten en mijn arm jeuken zodra ik begin te rennen.

You could also say:

• Zodra ik begin te rennen, voel ik mijn voorhoofd zweten en mijn arm jeuken.

That version puts the time relationship first: As soon as I start running...

Both are correct. The difference is mostly about emphasis and style.

Jeuken is the normal verb for to itch, especially for skin or a body part.

Examples:

• Mijn arm jeukt. = My arm itches.
• Mijn ogen jeuken. = My eyes itch.

It can also be used in figurative expressions, but in this sentence it is completely literal and physical.

A useful note: Dutch often talks about body parts as the subject of jeuken:

• Mijn arm jeukt.
• Mijn rug jeukt.

And after a perception verb:

• Ik voel mijn arm jeuken.

Because Dutch does not need a form of zijn here. The infinitive itself expresses the action being perceived.

So:

• Ik voel mijn voorhoofd zweten literally works like I feel my forehead sweat/sweating
• Ik voel mijn arm jeuken = I feel my arm itch/itching

English often uses -ing in this kind of sentence, but Dutch simply uses the infinitive.

Yes, you can, but the meaning changes slightly.

• zodra ik begin te rennen = as soon as I start running
• als ik begin te rennen = when I start running / if I start running, depending on context

So zodra stresses immediacy. If that is the idea you want, zodra is the better choice.

Yes, it is natural and understandable Dutch.

A native speaker might also phrase it in slightly different ways, for example:

• Zodra ik begin te rennen, voel ik mijn voorhoofd zweten en mijn arm jeuken.
• Als ik begin te rennen, begint mijn voorhoofd te zweten en gaat mijn arm jeuken.

These alternatives shift the structure a little, but the original sentence is perfectly normal. It uses a common Dutch pattern for directly describing physical sensations.