W tym miesiącu płacimy mniej za gaz.

Because w can take different cases depending on the meaning:

• w + locative = in / during
• w + accusative = into or motion toward the inside of something

Here, w tym miesiącu means in this month / this month, so Polish uses the locative.

• ten miesiąc = nominative/accusative
• tym miesiącu = locative

So:

• w tym miesiącu = in this month
• w ten miesiąc would not fit this meaning

Because after w in this time expression, the noun must be in the locative singular.

The noun miesiąc changes like this:

• nominative: miesiąc
• locative: miesiącu

So:

• ten miesiąc = this month as the basic dictionary form
• w tym miesiącu = in this month

This is a very common pattern with time expressions after w.

Tym agrees with miesiącu in case, number, and gender.

Since miesiącu is:

• singular
• masculine
• locative

the demonstrative ten must also appear in the locative singular masculine/neuter form:

• nominative: ten
• locative: tym

So:

• ten miesiąc
• w tym miesiącu

Polish adjectives and words like ten/ta/to have to match the noun.

Płacimy already includes the subject we.

In Polish, personal verb endings usually show who is doing the action, so you often do not need a separate subject pronoun.

• płacę = I pay
• płacisz = you pay
• płacimy = we pay

So my is optional:

• Płacimy mniej za gaz.
• My płacimy mniej za gaz.

Both are grammatical, but adding my gives extra emphasis, something like we are the ones paying less.

Because mniej is an adverb/comparative quantity word meaning less.

Here it modifies the idea of paying:

• płacimy mniej = we pay less

By contrast, mniejszy is an adjective meaning smaller and would describe a noun:

• mniejszy rachunek = a smaller bill

So:

• płacimy mniej = we pay less
• mniejszy gaz would not make sense here

In this sentence, płacić za + accusative means to pay for something.

So:

• płacić za gaz = to pay for gas

This is a very common construction:

• płacić za prąd = pay for electricity
• płacić za bilet = pay for a ticket
• płacić za obiad = pay for lunch

The preposition za requires the accusative here, and gaz has the same form in nominative and accusative singular, so you do not see a change.

They mean different things:

• płacić za coś = to pay for something
• płacić komuś = to pay someone

Examples:

• Płacę za kawę. = I’m paying for the coffee.
• Płacę kelnerowi. = I’m paying the waiter.

You can also combine them:

• Płacę kelnerowi za kawę. = I’m paying the waiter for the coffee.

In your sentence, the focus is on what we are paying for: gaz.

Usually gaz means gas in the sense of:

• natural gas
• gas supply
• gas used for heating/cooking

In everyday British English, gas matches well here.

If you mean petrol/gasoline for a car, Polish usually says:

• benzyna = petrol/gasoline
• sometimes gaz can refer to LPG/autogas, depending on context

But in płacimy mniej za gaz, the most natural meaning is we are paying less for gas as a utility bill.

Yes, Polish word order is flexible. This version sounds natural and puts the time expression first:

• W tym miesiącu płacimy mniej za gaz.

That feels like:

• This month, we’re paying less for gas.

Other possible orders:

• Płacimy mniej za gaz w tym miesiącu.
• Za gaz płacimy mniej w tym miesiącu.

They all mean roughly the same thing, but the emphasis changes:

• W tym miesiącu... emphasizes when
• Za gaz... emphasizes what for
• mniej naturally stays close to the verb because it modifies pay

The basic verb płacić is imperfective. It is used for:

• ongoing situations
• repeated actions
• general statements
• what is happening during a period of time

That fits this sentence well:

• W tym miesiącu płacimy mniej za gaz.
• This means something like during this month, our payment is lower

A perfective form would usually point to a completed act of paying, which is not the idea here.

Yes, but the meaning changes a bit.

• płacimy mniej = we are paying less / we pay less
• zapłacimy mniej = we will pay less / we’ll end up paying less

So:

• W tym miesiącu płacimy mniej za gaz. sounds like a present/current situation.
• W tym miesiącu zapłacimy mniej za gaz. sounds more like a future or single-result statement.

The original sentence is more natural if you are talking about the current bill or current pricing this month.

Yes. In natural English, we usually translate it as this month, not literally in this month.

So:

• W tym miesiącu płacimy mniej za gaz.
• natural English: This month, we’re paying less for gas.

Polish often uses a prepositional phrase where English just uses a simple time expression.

The main pronunciation points here are:

• ł in płacimy sounds like English w
• ą does not appear here
• cz in miesiącu sounds like ch in church, but a bit harder/cleaner
• c at the end of miesiąc becomes part of -cu in miesiącu, pronounced like tsu

A rough pronunciation guide:

• W tym miesiącu ≈ v tim myeh-SHYON-tsu
• płacimy ≈ pwa-CHEE-my
• mniej ≈ roughly mnyay
• za gaz ≈ za gas

Not perfect phonetics, but good enough to help an English speaker get started.