A tarifa de energia está mais cara este mês.

Because tarifa is a feminine noun in Portuguese, so it takes the feminine article a.

• a tarifa = the rate / tariff / fee
• o preço = the price
• a conta = the bill

In Portuguese, grammatical gender is part of the noun, and the article has to match it.

Not exactly. Tarifa usually means rate, tariff, or charge, not the physical bill itself.

So in this sentence:

• A tarifa de energia = the electricity rate / electricity charge

If you want to talk about the bill, you would more often say:

• a conta de energia = the electricity bill

So this sentence is focusing on the cost/rate of electricity, not necessarily the bill as a document.

Because tarifa de energia is a fixed-type noun phrase meaning energy rate or electricity rate.

Here, de works like of in English:

• tarifa de energia = energy rate
• conta de luz = electricity bill
• preço da energia = the price of the energy / electricity

Using da energia would sound more specific, as if you were talking about the price of the electricity in a particular context. But tarifa de energia is the normal general expression.

Because estar is often used for a current condition or changeable state, while ser is more for permanent or defining characteristics.

• está mais cara = is more expensive right now / has become more expensive
• é cara = is expensive in general

Since the sentence includes este mês and talks about a current situation, está is the natural choice.

Because cara is an adjective agreeing with the feminine noun tarifa.

Agreement in Portuguese matters:

• o preço está caro
• a tarifa está cara

Since tarifa is feminine singular, the adjective must also be feminine singular:

• cara

Mais cara means more expensive.

It is the comparative form:

• caro / cara = expensive
• mais caro / mais cara = more expensive

So:

• A tarifa está cara. = The rate is expensive.
• A tarifa está mais cara. = The rate is more expensive.

Often this implies comparison with an earlier time, even if that comparison is not stated directly.

Because this sentence does not explicitly say what it is being compared to. It just says the rate is more expensive this month, implying comparison with another time, usually last month or before.

If you wanted to say more expensive than last month, you would say:

• A tarifa de energia está mais cara do que no mês passado.

So do que is used for than when the comparison is made explicit.

Because time expressions are often placed at the end in Portuguese, just as they often are in English.

• A tarifa de energia está mais cara este mês.
• Este mês, a tarifa de energia está mais cara.

Both are correct. Putting este mês at the end sounds very natural and neutral.

If you put it first, it gives a little more emphasis to the time:

• Este mês, ... = This month, ...

Yes, but the meaning and tone can be slightly different.

• este mês = this month
• neste mês = in this month

In many situations, este mês sounds more natural here.
Neste mês can sound a little more specific or more formal, depending on context.

So for this sentence, este mês is the most natural everyday choice.

Yes. In Brazilian Portuguese, energia often means electrical power/electricity, especially in everyday contexts like bills, rates, outages, and utilities.

Examples:

• conta de energia = electricity bill
• tarifa de energia = electricity rate
• falta de energia = power outage

So although energia literally means energy, in this kind of sentence it naturally refers to electricity.

Yes, and people may say that in casual speech, but it is slightly less precise.

• A tarifa de energia está mais cara = the electricity rate is more expensive
• A energia está mais cara = electricity is more expensive

The second version is common and understandable, especially in conversation. The original sentence sounds a bit more formal or specific because it refers to the rate/charge itself.

In standard Brazilian pronunciation, está is roughly pronounced like eh-STAH, with stress on the second syllable.

A few points:

• the e at the start is a short vowel, like eh
• tá is stressed
• the final a sounds like ah

In casual speech, Brazilians very often shorten está to tá:

• A tarifa de energia tá mais cara este mês.

That is very common in spoken Brazilian Portuguese, though está is more formal or careful.