We wtorek mam mniej pracy niż w poniedziałek.

Polish uses we instead of w before some awkward consonant clusters, especially when pronunciation would be difficult.

So:

• we wtorek = correct
• w wtorek = not standard

The cluster w + wt- is hard to say smoothly, so Polish inserts e.

You see the same idea in some other phrases, for example:

• we Wrocławiu
• we Francji

So here, we wtorek is simply the normal form.

They are in the accusative.

With days of the week, Polish commonly uses:

• w/we + accusative = on Monday / on Tuesday / on Friday

So:

• we wtorek = on Tuesday
• w poniedziałek = on Monday

For these masculine inanimate nouns, the accusative looks the same as the nominative, so the form does not visibly change:

• nominative: wtorek

• accusative: wtorek

• nominative: poniedziałek
• accusative: poniedziałek

That is why the words look unchanged.

Because the verb already tells you the subject.

mam means I have, so Polish usually does not need the pronoun ja.

• mam = I have
• ja mam = I have, with extra emphasis or contrast

So the sentence naturally says:

• We wtorek mam mniej pracy...

If you added ja, it would sound more emphatic, for example if you were contrasting yourself with someone else:

• Ja mam mniej pracy niż ty.

Because mniej usually takes the genitive.

So:

• mniej pracy = less work

Here:

• praca is the dictionary form
• pracy is the genitive singular form

This is very common after words like:

• dużo
• mało
• więcej
• mniej

Compare:

• mam dużo pracy = I have a lot of work
• mam mało pracy = I have little work
• mam więcej pracy = I have more work
• mam mniej pracy = I have less work

Because mniej pracy and mniejsza praca mean different things.

• mniej pracy = less work
• mniejsza praca = a smaller job / a smaller piece of work

In your sentence, the meaning is about quantity, not about the physical or abstract size of one job. So mniej pracy is the correct choice.

An English speaker often wants to map less to an adjective like smaller, but Polish does not work that way here.

niż means than in comparisons.

So:

• mniej ... niż ... = less ... than ...

In this sentence:

• mam mniej pracy niż w poniedziałek
• literally: I have less work than on Monday

It is the normal word used after comparative expressions like:

• więcej niż = more than
• mniej niż = less than
• lepszy niż = better than

Not naturally in this sentence.

niż is the right choice because the comparison is followed by a time phrase:

• niż w poniedziałek = than on Monday

If you tried od poniedziałku, it would usually mean from Monday, which is a different meaning.

So:

• mam mniej pracy niż w poniedziałek = correct
• mam mniej pracy od poniedziałku = sounds like I have less work from Monday onward, not than on Monday

Yes, pracy is a very natural, neutral choice.

• praca = work
• pracy = of work / work, after mniej

Possible alternatives depend on tone:

• mam mniej roboty = I have less work
  This is common and colloquial.
• mam mniej zajęć = I have fewer duties/classes/activities
  This is not exactly the same as work.

So pracy is probably the safest and most standard choice.

No, Polish word order is flexible, but this sentence has a very natural order.

• We wtorek mam mniej pracy niż w poniedziałek.

This puts Tuesday first, which feels like the topic: As for Tuesday...

You could also say:

• Mam we wtorek mniej pracy niż w poniedziałek.

That is also natural.

The difference is mostly about emphasis, not basic meaning.

Very roughly:

• We wtorek mam... = focusing first on Tuesday
• Mam we wtorek... = focusing first on I have

It depends on context.

This sentence can mean either:

• this Tuesday, compared with Monday
• or a general pattern, depending on what has already been said

If you want to make the habitual meaning very clear, Polish often uses the plural:

• We wtorki mam mniej pracy niż w poniedziałki.
• On Tuesdays I have less work than on Mondays.

So the singular sentence can be understood from context, but the plural is clearer for repeated routine.

Because in Polish, days of the week are normally written with lowercase letters.

So:

• poniedziałek
• wtorek
• środa

This is different from English, where Monday and Tuesday are capitalized.

In Polish, you only capitalize them at the beginning of a sentence or in special title-style writing.

A few parts are especially worth noticing:

• we wtorek
  The w in Polish sounds roughly like English v.
  So we wtorek is roughly vev-TO-rek.

• mniej
  This is tricky for English speakers. It is roughly like mnyey, said as one syllable.

• niż
  Roughly like neezh, with ż sounding like the s in measure.

• poniedziałek
  Stress is on the second-to-last syllable:
  po-nie-DZIA-łek

Polish usually stresses the penultimate syllable, and that rule works here too:

• wTOrek
• po-nie-DZIA-łek

Because Polish does not have articles like English a and the.

So Polish simply says:

• we wtorek
• w poniedziałek
• mam mniej pracy

Context tells you whether something is definite, general, already known, and so on.

That is why a very natural Polish sentence can look shorter than its English equivalent.