Kiam mi sidas tro longe, mia korpo laciĝas, kaj mia piedo fariĝas malvarma.

Kiam means when.

It introduces a time clause:

• Kiam mi sidas tro longe = When I sit too long

In Esperanto, a clause beginning with kiam works much like when in English. After that clause, the main clause follows:

• mia korpo laciĝas, kaj mia piedo fariĝas malvarma

So the whole structure is:

• Kiam ... , ...
• When ... , ...

Mi sidas is the normal, simple way to say I am sitting / I sit.

Esperanto often uses the simple present tense where English uses either:

• I sit
• I am sitting

So:

• mi sidas can mean I sit or I am sitting, depending on context.

The longer form mi estas sidanta is possible, but it is more explicit and usually unnecessary unless you really want to emphasize the ongoing nature of the action.

Because longe is an adverb, and here it modifies the verb sidas.

• longa = long (adjective)
• longe = for a long time / long (adverb)

In the sentence, the idea is not that the sitting itself is a long thing as a noun-like object; rather, it describes how long the action lasts.

So:

• mi sidas tro longe = I sit too long / I sit for too long

This is the normal Esperanto way to express duration in this kind of sentence.

Tro means too much or too in the sense of excessively.

So:

• tro longe = too long
• tro rapide = too quickly
• tro varma = too warm

In this sentence, tro longe means the amount of time is excessive.

Laciĝas comes from:

• laca = tired
• laciĝi = to become tired / to get tired
• laciĝas = becomes tired / gets tired

The important part is -iĝ-, which often means to become or to undergo a change of state.

So:

• mia korpo laciĝas = my body gets tired

This is different from just estas laca:

• mia korpo estas laca = my body is tired
  (describes a state)
• mia korpo laciĝas = my body is getting tired / becomes tired
  (describes a change)

This is a very common Esperanto question.

• laciĝi = to become tired
• lacigi = to make someone/something tired

So:

• Mi laciĝas. = I get tired.
• Tio lacigas min. = That makes me tired.

In your sentence, mia korpo laciĝas means my body gets tired, not my body makes something tired. That is why -iĝ- is used, not -ig-.

Both are grammatical, but they feel slightly different.

• Mi laciĝas = I get tired
• Mia korpo laciĝas = My body gets tired

Using mia korpo puts special focus on the body itself. A learner may notice that English usually just says I get tired, and Esperanto often would too.

So this sentence is fine, but it is a little more physically focused than the simpler:

• Kiam mi sidas tro longe, mi laciĝas...

If the speaker wants to emphasize bodily discomfort, mia korpo laciĝas makes sense.

Yes, both can work, but they are slightly different in feel.

• fariĝas malvarma = becomes cold
• malvarmiĝas = gets cold / cools down

In many contexts, they are very close.

Fariĝi + adjective is a very common Esperanto pattern for become + adjective:

• fariĝas granda = becomes big
• fariĝas klara = becomes clear
• fariĝas malvarma = becomes cold

Meanwhile, malvarmiĝi is a more direct verb meaning to become cold.

So in this sentence:

• mia piedo fariĝas malvarma
• mia piedo malvarmiĝas

Both are understandable and natural. The first one emphasizes the change into the state malvarma.

Because malvarma is an adjective describing mia piedo.

• mia piedo = noun
• malvarma = adjective agreeing with that noun

So:

• mia piedo fariĝas malvarma = my foot becomes cold

If you used malvarme, that would be an adverb, and it would not fit this structure.

A useful comparison:

• La ĉambro estas malvarma. = The room is cold.
• Estas malvarme en la ĉambro. = It is cold in the room.

Because it is a predicate adjective, not a direct object.

In Esperanto, -n is used mainly for:

• direct objects
• direction in some expressions

But in:

• mia piedo fariĝas malvarma

malvarma is describing what mia piedo becomes. It is linked to the subject, not acting as an object.

The same happens in sentences like:

• Li estas laca. = He is tired.
• Ŝi fariĝis feliĉa. = She became happy.

No -n is used on laca or feliĉa, and likewise none on malvarma here.

Yes, English often says my feet in this kind of situation, but Esperanto can use either singular or plural depending on what the speaker means.

• mia piedo = my foot
• miaj piedoj = my feet

If only one foot is being referred to, singular is correct. If both feet get cold, then you would say:

• miaj piedoj fariĝas malvarmaj

So the singular here is not a grammar issue; it is just a choice of meaning.

In this sentence, repeating mia is normal and clear:

• mia korpo
• mia piedo

Esperanto often repeats possessives where English might also repeat them. You generally should not omit mia before piedo unless the meaning is still completely clear from a different structure.

So this is good style for a learner:

• mia korpo laciĝas, kaj mia piedo fariĝas malvarma

It is explicit and easy to understand.

Because Kiam mi sidas tro longe is a subordinate clause placed before the main clause.

So the comma separates:

• the when-clause
• the main statement

This is similar to English:

• When I sit too long, my body gets tired...

The comma is standard and helps readability.

Because Esperanto keeps the tense consistent with the time being described.

The sentence describes what generally happens whenever this situation occurs:

• When I sit too long, my body gets tired, and my foot becomes cold.

That is a general present-time truth or habit, so present tense is used throughout:

• sidas
• laciĝas
• fariĝas

Esperanto present tense often covers both:

• habitual actions
• general truths
• actions happening in the present context

Kaj means and.

It connects the two main results:

• mia korpo laciĝas
• mia piedo fariĝas malvarma

So the structure is:

• When I sit too long, X happens, and Y happens.

Very straightforward:

• kaj = and

The letter ĝ is pronounced like the j in judge.

So roughly:

• laciĝas ≈ la-TSEE-jas
• fariĝas ≈ fa-REE-jas

A few helpful points:

• c in Esperanto is always like ts
• ĝ is like English j
• stress is always on the second-to-last syllable

So:

• la-ci-ĜAS
• fa-ri-ĜAS

Not with exactly the same meaning.

• kiam = when
• se = if

So:

• Kiam mi sidas tro longe... = When I sit too long...
  This suggests something that does happen, or happens regularly.

• Se mi sidas tro longe... = If I sit too long...
  This is more conditional or hypothetical.

Both can be possible depending on what you want to say, but they are not identical. In your sentence, kiam suggests a repeated real-life pattern.