Όταν έχω συνάχι και πονοκέφαλο μαζί, μένω σπίτι και ξεκουράζομαι.

Here Όταν means when or, more naturally in this kind of sentence, whenever.

So Όταν έχω... introduces a time clause:

• Όταν έχω συνάχι και πονοκέφαλο μαζί...
• When/Whenever I have a cold and a headache at the same time...

In Greek, όταν is very common for this kind of repeated situation.

Because the sentence describes a usual/repeated action, not one specific event happening only once.

Greek often uses the present tense for habits or general truths:

• Όταν έχω... μένω... ξεκουράζομαι.
• Whenever I have..., I stay..., I rest.

So this is not necessarily I am sick right now. It means something like this is what I do in that situation.

Because Greek usually does not need subject pronouns when the verb ending already shows who the subject is.

Here:

• έχω = I have
• μένω = I stay
• ξεκουράζομαι = I rest

So Greek can leave out εγώ (I) because it is already clear.

If you added εγώ, it would usually sound more emphatic:

• Εγώ μένω σπίτι...
• I stay home... / As for me, I stay home...

Because πονοκέφαλος is the dictionary form of the noun, that is, the nominative singular.

After έχω (I have), the noun is the direct object, so Greek uses the accusative:

• nominative: ο πονοκέφαλος = the headache
• accusative: τον πονοκέφαλο = the headache as an object

So:

• έχω πονοκέφαλο = I have a headache

This is very normal Greek grammar.

Because συνάχι is a neuter noun, and in the singular its nominative and accusative forms are the same.

So:

• nominative: το συνάχι
• accusative: το συνάχι

That is why after έχω it still stays συνάχι.

By contrast, πονοκέφαλος is masculine, so it changes:

• πονοκέφαλος → πονοκέφαλο

In Greek, after έχω, common illnesses and symptoms are often used without the article.

So these are very natural:

• έχω συνάχι
• έχω πονοκέφαλο
• έχω πυρετό

This works a lot like English:

• I have a cold
• I have a headache

If you used an article, it could sound more specific or more definite in context, but the article-less version is extremely common and natural here.

Μαζί here means together or at the same time.

So:

• συνάχι και πονοκέφαλο μαζί means
• a cold and a headache together
• both at the same time

The sentence would still make sense without μαζί, but μαζί makes the idea of simultaneously clearer and stronger.

Both are possible, but μένω σπίτι is a very natural, idiomatic way to say I stay home.

There is a small difference in feel:

• μένω σπίτι = I stay home
• μένω στο σπίτι = I stay at home / in the house

The version without the article and preposition often feels more like the general idea of home as a state or destination, similar to English go home vs. go to the house.

So in this sentence, μένω σπίτι is exactly the kind of Greek a native speaker would naturally use.

Ξεκουράζομαι is a verb in the middle/passive-looking form (-ομαι), but here it has an active meaning: I rest or I get some rest.

This is very common in Modern Greek. Not every -ομαι verb is truly passive in meaning.

Compare:

• ξεκουράζω = I rest someone/something
• ξεκουράζομαι = I rest / I get rested / I get some rest

So in this sentence, ξεκουράζομαι is simply the normal way to say I rest.

Because Όταν έχω συνάχι και πονοκέφαλο μαζί is a subordinate time clause, and it comes before the main clause.

Then the main clause follows:

• μένω σπίτι και ξεκουράζομαι

Greek usually puts a comma after an introductory subordinate clause, just as English often does:

• When I have..., I stay home...

So the comma is standard punctuation here.

Because it is joining two different pairs of things.

First και joins the two symptoms:

• συνάχι και πονοκέφαλο
• a cold and a headache

Second και joins the two actions:

• μένω σπίτι και ξεκουράζομαι
• I stay home and rest

This is completely normal Greek. Just like in English, you use and wherever you need to connect words or clauses.