Kosilica opet ne radi dobro, pa danas nećemo pokositi travu.

Because raditi here means to work / to function, and in the present tense Croatian negates it with ne + verb:

• radi = it works
• ne radi = it does not work / it isn’t working

So:

• Kosilica ne radi dobro = The lawnmower isn’t working well

You would not use nije here, because nije is the negative form of je (is), not of radi.

Opet means again.

In this sentence:

• Kosilica opet ne radi dobro = The lawnmower again isn’t working well / The lawnmower isn’t working well again

Its position is fairly natural here, between the subject and the rest of the predicate. Croatian word order is flexible, so you may also hear:

• Opet kosilica ne radi dobro
• Kosilica ne radi opet dobro

But Kosilica opet ne radi dobro is a very normal, neutral choice.

Because dobro is an adverb, not an adjective.

Here it modifies the verb radi (works / functions), so it means well, not good:

• dobar / dobra / dobro = good as an adjective
• dobro = also well as an adverb, depending on context

So:

• Kosilica je dobra = The lawnmower is good
• Kosilica radi dobro = The lawnmower works well

In your sentence, the meaning is about how it works, so the adverb dobro is correct.

Here pa means something like:

• so
• therefore
• and so
• which means

It connects the first clause to the result:

• Kosilica opet ne radi dobro, pa danas nećemo pokositi travu.
• The lawnmower isn’t working well again, so today we won’t mow the grass.

In everyday Croatian, pa is very common and often sounds more natural than a more formal connector like zato or stoga in casual speech.

Nećemo pokositi is the negative future tense.

It is formed with:

• nećemo = we will not
• infinitive: pokositi = to mow

So:

• nećemo pokositi = we will not mow

This comes from the verb htjeti (to want), which is used to form the future:

• ćemo = we will
• nećemo = we will not

Compare:

• pokosit ćemo travu = we will mow the grass
• nećemo pokositi travu = we will not mow the grass

In Croatian, the negative future usually keeps the auxiliary and infinitive separate like this.

This is a question of aspect, which is very important in Croatian.

• kositi = imperfective: mowing as a process, repeated action, ongoing action
• pokositi = perfective: to mow something completely, to finish mowing

In this sentence, the idea is we won’t get the grass mowed / we won’t mow it today, so the perfective pokositi fits very well.

Compare:

• Danas kosimo travu. = Today we are mowing the grass / we mow grass today
  (focus on the activity)
• Danas ćemo pokositi travu. = Today we will mow the grass
  (focus on completing it)

So nećemo pokositi travu suggests we won’t manage to mow the grass / we won’t get it done.

Because travu is the accusative singular form of trava (grass).

The verb pokositi takes a direct object, and direct objects are usually in the accusative:

• nominative: trava = grass
• accusative: travu = grass as the object

So:

• Trava je mokra. = The grass is wet.
  (trava = subject)
• Pokosit ćemo travu. = We will mow the grass.
  (travu = object)

This is a very common pattern in Croatian.

Yes, danas (today) could go in other places. Croatian word order is flexible.

The sentence has:

• ..., pa danas nećemo pokositi travu.

This sounds natural and puts a little emphasis on today.

Other possible versions:

• ..., pa nećemo danas pokositi travu.
• ..., pa travu danas nećemo pokositi.

These are all possible, but they may shift emphasis slightly. The original version is neutral and very natural.

Kosilica usually means mower, and in everyday context it often means lawnmower.

Because the sentence later says pokositi travu (mow the grass), it is clearly a lawnmower here.

Croatian often relies on context for this kind of thing. In another context, kosilica could be understood more generally as a cutting/mowing machine, but in ordinary household speech it usually means lawnmower.

Croatian has no articles like a/an/the.

So:

• kosilica can mean a lawnmower or the lawnmower
• trava / travu can mean grass or the grass

You understand which one is meant from context.

In this sentence, English naturally uses the because both things are specific in the situation:

• The lawnmower isn’t working well again, so today we won’t mow the grass.

But Croatian expresses that without an article.

Yes. Ne radi dobro can be translated in several natural ways, depending on context:

• isn’t working well
• isn’t working properly
• doesn’t work well
• is malfunctioning (a bit more formal)

So the Croatian is simple and idiomatic, while English has a few good options.

Nećemo is pronounced approximately nye-cheh-mo, though that is only a rough guide.

A few points:

• ne sounds like ne in Nell, but shorter
• ć is a soft sound, somewhat like a very soft ch
• e is a clear short e
• stress can vary by dialect, but learners should focus first on getting the sounds right

The same soft ć appears in pokositi?
No — pokositi has ti, not ći. So the special sound is only in nećemo here.

For learners, a good practical approximation is:

• nećemo ≈ nye-cheh-mo

Even if your ć is not perfect at first, you will still be understood.

Yes, absolutely.

• ne radi dobro = doesn’t work well
• ne radi kako treba = doesn’t work as it should / isn’t working properly

Both are natural. The version with kako treba can sound a bit more idiomatic in some situations when talking about a machine that is not functioning correctly.

So these are both good:

• Kosilica opet ne radi dobro.
• Kosilica opet ne radi kako treba.

The first is simpler; the second can sound slightly more specific about proper functioning.