Ništa ne jedem sada, jer nemam vremena.

Yes, ništa means nothing, but Serbian normally uses negative concord. That means a negative word like ništa, niko, or nikad goes together with a negative verb.

So:

• Ništa ne jedem = I’m not eating anything / I eat nothing
• not Ništa jedem — that would be incorrect

This is very normal in Serbian and other Slavic languages.

It can mean both, depending on context.

Serbian does not have a separate tense like English I am eating. The present tense jedem can mean:

• I eat
• I am eating

In this sentence, sada makes it clear that the meaning is right now, so Ništa ne jedem sada means I’m not eating anything now.

Because Serbian verb endings usually already show the subject.

• jedem = I eat / I am eating
• nemam = I do not have

So ja is unnecessary unless you want extra emphasis or contrast.

For example:

• Ja ništa ne jedem sada = I’m not eating anything now
  This sounds more emphatic, like I’m the one who isn’t eating now.

Yes. Both are possible and natural.

Serbian word order is more flexible than English. The difference is mostly about emphasis:

• Ništa ne jedem sada puts a little more focus on nothing
• Ne jedem ništa sada is also very normal and may sound slightly more neutral to some speakers

You may also hear:

• Sad ne jedem ništa
• Ne jedem ništa sad

All of these can work, depending on rhythm and emphasis.

This is about aspect.

• jesti is imperfective: eating as an ongoing or general action
• pojesti is perfective: to eat something up, to finish eating it

Since the sentence describes what is happening now as a current situation, Serbian uses the imperfective verb:

• Ništa ne jedem sada

Using pojedem here would not fit well, because perfective verbs are generally not used for ongoing present actions in this way.

Here ništa is the direct object of jedem, so it is in the accusative.

With ništa, the accusative form looks the same as the nominative, so you do not see a form change.

So in:

• Ništa ne jedem

ništa is functioning as what I am not eating.

Because Serbian normally says imati vremena / nemati vremena for to have time / not to have time.

Here vremena is the genitive singular form of vreme.

This expression is very standard and idiomatic:

• Imam vremena = I have time
• Nemam vremena = I don’t have time

Using nemam vreme for this meaning would sound wrong or unnatural in standard Serbian.

A useful way to think about it is that vremena here refers to some amount of time available, not time as a general abstract concept.

Jer means because.

So:

• jer nemam vremena = because I don’t have time

Yes, you could also say:

• zato što nemam vremena

Both mean because I don’t have time.
Very roughly:

• jer is shorter and very common
• zato što is also common and can sound a bit fuller or more explicit

In this sentence, both are fine.

Yes. Both mean now.

• sada = full form
• sad = shorter everyday form

So these mean the same thing:

• Ništa ne jedem sada
• Ništa ne jedem sad

In casual speech, sad is extremely common.