Learn Hardcore Czech: Když je můj syn nemocný, jsem smutný.

Když means when in the sense of whenever / every time / when(ever) this happens, that happens.
The sentence says: When my son is ill, I am sad (in general). It describes a repeated or typical situation.

když = when(ever) (temporal, general condition): Když je můj syn nemocný, jsem smutný.
kdy = when? (question word): Kdy je tvůj syn nemocný? – When is your son ill?
jestli = if / whether (for real conditions or indirect questions):
Jestli je můj syn nemocný, zůstanu doma. – If my son is ill, I’ll stay at home.

You can’t replace když with kdy in this sentence, and jestli would change the meaning from “when(ever)” to “if”.

Why is there a comma before jsem smutný? Is it required?

Yes, the comma is required in Czech.

Když je můj syn nemocný is a subordinate clause (introduced by když).
jsem smutný is the main clause.

In Czech, you almost always put a comma between a subordinate clause and the main clause:

Když je můj syn nemocný, jsem smutný.
You can also flip the order: Jsem smutný, když je můj syn nemocný. (comma still needed)

Unlike in English, you do not drop the comma just because the když-clause comes second.

Why is the word order Když je můj syn nemocný and not Když můj syn je nemocný or Když je nemocný můj syn?

All three are grammatically possible, but they sound different in terms of emphasis and naturalness.

Neutral, most natural: Když je můj syn nemocný, jsem smutný.
(subject můj syn before adjective nemocný)
Když můj syn je nemocný, … – grammatically OK, but sounds a bit marked / emphatic, like stressing můj syn or contrasting with someone else’s child.
Když je nemocný můj syn, … – puts emphasis on můj syn, as in: When it’s *my son who is ill (not someone else’s)*.

Basic Czech word order in simple statements is similar to English: Subject–Verb–(Complement). Variations are used mostly for emphasis and information structure, not to change grammar.

Why is můj syn in this form and not something like mého syna?

Můj syn is in the nominative case, because it is the subject of the sentence — the thing that is ill.

můj syn – nominative singular (subject)
mého syna – would be accusative or genitive; here that would be incorrect, because nothing is being “done to” the son.

The pattern is:

Kdo je nemocný? – Who is ill? → Můj syn je nemocný.
So in the larger sentence: Když je můj syn nemocný, jsem smutný.
The inner clause is still just: Můj syn je nemocný.

Do I always need to say můj with family members, or can I just say syn?

You don’t always have to use the possessive pronoun.

In many contexts, especially when it’s clear from context whose family member you mean, you can omit můj:

Když je syn nemocný, jsem smutný. – When (my) son is ill, I’m sad.
Listeners will normally understand it as my son if you’re talking about your own family.

However, můj syn is perfectly natural and common, especially:

when introducing new information: Můj syn je nemocný.
when you need to contrast ownership: Můj syn je nemocný, ale tvůj je zdravý.

So: both Když je syn nemocný and Když je můj syn nemocný are fine; the latter is just more explicit.

Why does nemocný end in -ý here?

Nemocný is an adjective meaning ill / sick, and it must agree with the gender, number and case of the noun syn.

syn is masculine, singular, nominative.
The basic masculine nominative singular adjective ending is -ý (or -í in some adjectives).

So:

můj syn je nemocný – my (masculine) son is ill
If it were a daughter (dcera, feminine), it would be:
Moje dcera je nemocná. (feminine ending -á)

In your sentence, můj syn is masculine singular, so nemocný must also be masculine singular.

Does jsem smutný change if the speaker is a woman?

The verb stays the same, but the adjective changes.

A man says: Jsem smutný.
A woman says: Jsem smutná.

The form of být (to be) — jsem — already shows 1st person singular (“I”), but it does not show gender. The adjective smutný/smutná/smutné carries gender:

masculine: smutný
feminine: smutná
neuter: smutné

So a woman would say:
Když je můj syn nemocný, jsem smutná.

Why is it smutný and not smutno? What’s the difference?

Both exist, but they are different constructions:

Jsem smutný. – literally I am sad.
- Full adjective, agrees with your gender.
- Masculine speaker: smutný, feminine: smutná.
Je mi smutno. – literally It is sad to me / I feel sad.
- smutno is an adverb-like form.
- Does not change for gender; both men and women say Je mi smutno.

You can rephrase your sentence in a more idiomatic way like:

Když je můj syn nemocný, je mi smutno. – When my son is ill, I feel sad.

Both are correct; je mi smutno sounds slightly more natural in everyday speech.

Could I say Když bude můj syn nemocný, budu smutný? What’s the difference?

Yes, that’s correct, but it’s more clearly future.

Když je můj syn nemocný, jsem smutný.
General truth / repeated situations: Whenever my son is ill, I’m sad.
Když bude můj syn nemocný, budu smutný.
Talking about a future situation: When my son is (will be) ill, I will be sad.

Czech often uses the present tense like English does for general conditions. For specific future situations, you normally use bude / budu.

Can I drop je or jsem, like in some languages, and say Když můj syn nemocný, smutný?

In standard Czech, you should not drop the forms of být (to be) in this sentence.

Correct standard forms:

Když je můj syn nemocný, jsem smutný.

In informal speech, people sometimes omit je in 3rd person present:

Very colloquial: Když můj syn nemocnej, jsem smutnej.
(with both grammar and pronunciation made colloquial)

But this is not standard and not appropriate for writing or for learners.

You definitely cannot drop jsem in the main clause; you must say jsem smutný / jsem smutná.

How do you pronounce jsem and where is the stress in this sentence?

Pronunciation:

jsem is pronounced roughly like [sem]; the j is usually not clearly pronounced.
Když – [gdiž] (the k often assimilates to g before d, so it sounds close to gdish).
syn – like English sin.
můj – roughly mooy, with a long ú/ů sound.
nemocný – neh-mots-ny (stress on ne).
smutný – smoot-ny (u as in “put”, not “boot”; stress on smut).

Czech stress is almost always on the first syllable of each word (or of each “clitic group”), not on the important word like in English. So each content word here is stressed on its first syllable.

Is there any difference in meaning between Když je můj syn nemocný, jsem smutný and Jsem smutný, když je můj syn nemocný?

Both are correct and mean practically the same thing.

Když je můj syn nemocný, jsem smutný.
Starts with the condition, then gives the result. Slightly more neutral.
Jsem smutný, když je můj syn nemocný.
Starts with your emotional state, then explains under what condition. Slightly more focused on being sad.

The difference is subtle and mostly about emphasis / flow, not grammar.

Could I use a different verb like onemocní instead of je nemocný?

Yes, and it changes the nuance slightly.

Když je můj syn nemocný, jsem smutný.
Focus on the state of being ill.
Když můj syn onemocní, jsem smutný.
onemocní = falls ill / gets sick (perfective verb, the moment of becoming ill).
This focuses more on the event of him getting sick.

Another natural version:

Když můj syn onemocní, je mi smutno. – When my son falls ill, I feel sad.

Both are common; onemocní is event-focused; je nemocný is state-focused.

If my child is a daughter, how would the whole sentence change?

You must change both the noun and the adjectives to feminine forms.

For a daughter (dcera, feminine):

Když je moje dcera nemocná, jsem smutný. – said by a man
Když je moje dcera nemocná, jsem smutná. – said by a woman

Changes:

můj syn → moje dcera (possessive pronoun also has feminine form moje)
nemocný → nemocná (agrees with dcera)
smutný / smutná – still agrees with the speaker’s gender, not with the daughter.

So you always have two separate agreements:

child + nemocný/nemocná (agrees with the child)
I + smutný/smutná (agrees with the speaker).