Learn Hardcore Czech: Ten oběd není špatný, ale není moc zdravý. - That lunch is not bad, but it is not very healthy.

oběd is masculine singular (nominative).
The masculine singular form is ten.
to is the neuter form, so to oběd is grammatically wrong as a normal noun phrase.

You can drop the demonstrative and say:

Oběd není špatný, ale není moc zdravý.

This sounds a bit more general (talking about “lunch” in general), while ten oběd points to a particular lunch you’re talking about (“that lunch” / “this lunch”).

You could also hear Tento oběd (more emphatic, often more formal) or colloquial Tenhle oběd (“this lunch here”).

How do I know that oběd is masculine, and how does that influence špatný and zdravý?

You normally learn the gender with the noun; dictionaries mark oběd m. for “masculine”.
A quick rule of thumb: nouns ending in a consonant are often masculine.

Because oběd is masculine singular nominative (it’s the subject), the adjectives that describe it must agree:

masculine: špatný, zdravý
feminine: špatná, zdravá
neuter: špatné, zdravé

Compare:

Ten oběd není špatný. – That lunch (m.) is not bad.
Ta večeře není špatná. – That dinner (f.) is not bad.
To jídlo není špatné. – That food (n.) is not bad.

So the -ý endings are there because of masculine singular agreement with oběd.

Why is it špatný and zdravý, not špatně and zdravě?

Špatný and zdravý are adjectives.
Špatně and zdravě are adverbs.

After the verb být (je / není = “is / is not”), you normally use an adjective in Czech, because you are describing the state/quality of the noun:

oběd je špatný – the lunch is bad
oběd je zdravý – the lunch is healthy

Adverbs like špatně, zdravě describe how something is done:

Vaří špatně. – He/She cooks badly.
Jí se tu zdravě. – People eat healthily here.

So in this sentence you must use the adjective forms špatný, zdravý.

How is není formed, and what would the positive form be?

Není is the 3rd person singular negative of být (“to be”).

Present tense of být (colloquial normal forms):

já: jsem – nejsem
ty: jsi – nejsi
on/ona/ono: je – není
my: jsme – nejsme
vy: jste – nejste
oni: jsou – nejsou

So the positive version of the sentence would be:

Ten oběd je špatný, ale je moc zdravý.
(“That lunch is bad, but it is very healthy.” – odd meaning, but grammatically correct.)

In our original sentence, both clauses are in the negative:

není špatný – is not bad
není moc zdravý – is not very healthy / not very good for you

Why don’t we say something like Ono není špatný for “It is not bad”?

Czech usually does not use a separate pronoun for “it” when the subject is already named.

The subject here is ten oběd.
So Ten oběd není špatný already contains the subject. Adding on or ono would be redundant.

You can say something like:

Ten oběd, ten není špatný.
On ten oběd není špatný.

These versions are more emphatic or conversational, a bit like “That lunch, that’s not bad” or “That lunch, it really isn’t bad”. In normal neutral wording, you just say Ten oběd není špatný without on/ono.

What exactly does není moc zdravý mean? Does moc here mean “very” or “too”?

In negative sentences, moc most often means “very much / very”, so:

není moc zdravý ≈ “is not very healthy” / “is not very good for you”.

In positive sentences, moc is flexible:

Je moc zdravý.
– in everyday speech this can mean “very healthy”, and in some contexts it can also suggest “too healthy”.

But with není, Czech speakers normally understand:

Ten oběd není moc zdravý.
= “That lunch isn’t very healthy.”

It does not usually mean “is not too healthy” in the English sense “is actually unhealthy”; it’s more like “it’s somewhat unhealthy / not very healthy”.

Why is moc before zdravý? Can I say zdravý moc?

Adverbs like moc, velmi, hodně usually come before the adjective they modify:

moc zdravý
velmi zdravý
hodně zdravý

Putting moc after the adjective (zdravý moc) is normally wrong or at least very marked and unnatural in this kind of sentence.

So:

Ten oběd není moc zdravý. – correct, natural
Ten oběd není zdravý moc. – sounds wrong/foreign.

Can we say Ten oběd není špatný, ale moc zdravý není? How is that different?

Yes, that sentence is completely correct:

Ten oběd není špatný, ale moc zdravý není.

Meaning is basically the same: “That lunch isn’t bad, but it’s not very healthy.”

The difference is in emphasis and rhythm:

… ale není moc zdravý. – normal, neutral order.
… ale moc zdravý není. – puts extra focus on “not very healthy”, almost like: “… but healthy it certainly isn’t.”

In speech, the second version can sound more contrastive or slightly more expressive.

Could we use zlý instead of špatný here, like Ten oběd není zlý?

You can say Ten oběd není zlý, and people will understand you.
However, the nuances are slightly different:

špatný – generally “bad, poor (quality), not good”. Very common with things, situations, food.
zlý – more “evil, mean, nasty”; with food it can mean “nasty-tasting, not good”.

So:

Ten oběd není špatný. – neutral: “The lunch isn’t bad.”
Ten oběd není zlý. – more like “The lunch isn’t bad at all / it’s kind of OK”, and often more colloquial, with a slightly different feel.

For safe, neutral “not bad (quality)”, špatný is the best default choice.

How would the sentence change with a feminine or neuter noun instead of oběd?

All the words that agree with the noun would change form.

Feminine noun, e.g. večeře (dinner, f.):

Ta večeře není špatná, ale není moc zdravá.
(ta – feminine demonstrative, špatná, zdravá – feminine adjectives)

Neuter noun, e.g. jídlo (food, meal, n.):

To jídlo není špatné, ale není moc zdravé.
(to – neuter demonstrative, špatné, zdravé – neuter adjectives)

This shows the gender agreement pattern the original sentence is using with masculine oběd:

masculine: ten oběd … špatný … zdravý
feminine: ta večeře … špatná … zdravá
neuter: to jídlo … špatné … zdravé

Why is there a comma before ale?

Czech punctuation puts a comma between coordinate clauses joined by ale (“but”):

Ten oběd není špatný, ale není moc zdravý.

Each part has its own verb (není / není), so they are two separate clauses:

Ten oběd není špatný
(ten oběd) není moc zdravý

Joined by ale, they must be separated by a comma. In normal written Czech, you should always write the comma in this structure.