Learn Hardcore Czech: Můj kolega má někdy skvělý nápad, ale další den říká, že je ten nápad příliš náročný. - My colleague sometimes has a great idea, but the next day he says that the idea is too demanding.

The possessive must match the grammatical gender, so we use můj, not moje.
Kolega is in the nominative singular here (it’s the subject), so the base form kolega is used, not kolegy, kolegu, etc.

What exactly does někdy mean, and could I also use občas? Does its position in the sentence matter?

Někdy means sometimes / at times.

You can often replace it with občas (also “sometimes, occasionally”). The nuance is small:

někdy – neutral “sometimes”
občas – often feels a bit more like “from time to time, every now and then”

Word order is quite flexible. All of these are acceptable:

Můj kolega má někdy skvělý nápad.
Můj kolega někdy má skvělý nápad.

The most usual in everyday speech is what you see: má někdy skvělý nápad.

Why is it má skvělý nápad and not something like dostane nápad or napadne ho nápad?

Czech commonly uses mít nápad = to have an idea, just like English.

Other options exist but have slightly different flavours:

dostane nápad – “gets an idea”, often for a sudden moment of inspiration
napadne ho nápad – literally “an idea occurs to him”, again stressing the suddenness
přijde s nápadem – “comes up with an idea”, focuses on proposing an idea

In your sentence, we’re talking about a neutral, repeated situation, so má skvělý nápad is the most natural.

Could I say mívá skvělý nápad instead of má někdy skvělý nápad? What is the difference?

Yes, you could say:

Můj kolega mívá skvělý nápad.
Or better: Můj kolega mívá skvělé nápady. (“My colleague tends to have great ideas.”)

Mít vs mívát:

má – simple present; can already express a habitual action, especially with někdy
mívá – explicitly habitual (“tends to have; has repeatedly / regularly”)

So má někdy skvělý nápad = sometimes he has a great idea.
mívá skvělé nápady would stress a clear pattern: he regularly comes up with great ideas.

Is skvělý nápad the usual way to say “great idea”? Could I also say dobrý nápad?

Both are correct, but the meaning differs slightly:

skvělý nápad – “great / brilliant idea”, clearly positive, stronger praise
dobrý nápad – “good idea”, positive but more neutral

So if in English you would say “That’s a great idea!”, To je skvělý nápad! is the closest equivalent.

In your sentence, skvělý nápad matches the English “great idea” well.

Why is there a comma before ale?

In Czech, you always put a comma before ale (“but”) when it connects two clauses or parts of a sentence:

… skvělý nápad, ale další den říká…

This is a standard punctuation rule, even if the first part is short.
So: comma + ale is normal and expected in written Czech.

Why is there no preposition before další den? Why not v dalším dni?

Další den is in the accusative and functions as a time expression (“the next day”). Czech often uses bare accusative for time:

každý den – every day
příští týden – next week
minulý rok – last year
další den – the next day

V dalším dni would be grammatically possible but sounds unnatural here and overly literal (“in the next day”). For “the next day” in narrative or description, další den (without a preposition) is the standard form.

Does další den always mean “the next day”, or can it also mean “another day”?

It can mean both, depending on context:

the next day (the day immediately after something)
another day (some other day, not today/now)

In your sentence, with a contrast “has a great idea, but the next day he says…”, the meaning is clearly the next day (the following day after the idea).

Why do we use říká, že… and not řekne, že…? What is the difference?

The difference is aspect:

říká – imperfective, present tense; here it describes a typical / repeated behaviour (“he says (that)… (on such occasions)”)
řekne – perfective, usually future or a single event (“he will say / he says once (at a specific moment)”)

Your sentence talks about what he sometimes does in general:
Můj kolega má někdy skvělý nápad, ale další den říká… – this is a habitual pattern, so říká is correct.

For a one‑off story in the past, you might say:

Měl skvělý nápad, ale další den řekl, že…

Why is the word order že je ten nápad příliš náročný and not že ten nápad je příliš náročný? Are both correct?

Both orders are grammatically correct:

že je ten nápad příliš náročný
že ten nápad je příliš náročný

Czech word order is relatively flexible. Very roughly:

že ten nápad je… feels a bit more neutral and close to English “that the idea is…”.
že je ten nápad… puts slight focus on the state / property (the “being too demanding”), but in everyday speech the difference is small.

You will hear both. The version in your sentence is natural and common.

Why repeat ten nápad? Could I just say …říká, že je příliš náročný?

Yes, you can absolutely say:

…ale další den říká, že je příliš náročný.

Here, je and náročný clearly refer back to nápad, so the noun can be omitted. Czech often drops subjects when they are obvious from context.

Repeating ten nápad:

že je ten nápad příliš náročný adds emphasis: that that (very) idea is too demanding.
The demonstrative ten also makes it feel slightly more pointed or contrastive: that idea we talked about.

You would not use an object pronoun here (e.g. že je on příliš náročný for the idea), because the idea is the subject of “is”, not an object.

What is the nuance of ten in ten nápad? Does it mean “the” or “that”?

Czech ten can correspond to English the, that, or sometimes this, depending on context.

In ten nápad here, it signals a specific, known idea – roughly “that idea (we just mentioned)” or “that particular idea”. It makes the reference clearer and a bit more emphatic than just nápad alone.

You could say:

…že je ten nápad příliš náročný. – that that idea is too demanding
…že je nápad příliš náročný. – also possible, a bit less pointed, less contrastive

What is the difference between příliš náročný, moc náročný, and příliš těžký?

All can translate as “too difficult / too demanding”, but with nuances:

příliš náročný – neutral, slightly more formal; “too demanding” (requires too much time/effort/resources)
moc náročný – common in speech; moc = “too / very”, more colloquial feeling
příliš těžký – literally “too heavy/too hard”, focusing more on difficulty than on demands like time, energy, money

In your context, příliš náročný nicely suggests that the idea would be too demanding to carry out, not just intellectually hard.