Eu mănânc mai puțin seara.

Yes. Romanian often drops subject pronouns because the verb ending already shows who is doing the action.

So:

• Eu mănânc mai puțin seara.
• Mănânc mai puțin seara.

both mean the same thing: I eat less in the evening.

Including eu adds emphasis or contrast, for example:

• Eu mănânc mai puțin seara, dar fratele meu mănâncă mult.
  I eat less in the evening, but my brother eats a lot.

Mănânc is the 1st person singular present tense of the verb a mânca (to eat).

Here are some present tense forms:

• eu mănânc = I eat
• tu mănânci = you eat
• el/ea mănâncă = he/she eats
• noi mâncăm = we eat
• voi mâncați = you all eat
• ei/ele mănâncă = they eat

So mănânc specifically means I eat.

Mai puțin means less.

In this sentence, it works like an adverbial expression modifying the verb:

• mănânc mai puțin = I eat less

A useful comparison:

• mai mult = more
• mai puțin = less

Examples:

• Mănânc mai mult dimineața. = I eat more in the morning.
• Beau mai puțină cafea. = I drink less coffee.

This is a very common question.

• seară = evening (the basic noun form)
• seara can mean the evening or, in many time expressions, in the evening / at nightfall

In this sentence, seara is being used in a time expression, so it means in the evening.

Similar time expressions exist in Romanian:

• dimineața = in the morning
• după-amiaza = in the afternoon
• seara = in the evening
• noaptea = at night

So seara here is the natural Romanian way to express in the evening.

Yes, grammatically it is the definite form:

• o seară = an evening
• seara = the evening

But in expressions of time, Romanian often uses this definite form where English uses a preposition:

• dimineața = in the morning
• seara = in the evening

So even though it literally looks like the evening, you should often understand it idiomatically as in the evening.

The word order is very natural:

• Eu = subject
• mănânc = verb
• mai puțin = adverbial phrase
• seara = time expression

So:

Eu mănânc mai puțin seara.

Romanian word order is fairly flexible, but this version is neutral and common.

You may also hear:

• Seara mănânc mai puțin. = In the evening, I eat less.

This puts more focus on seara.

Usually, mănânc mai puțin is understood as I eat less in quantity or amount.

If you specifically want to say I eat fewer foods / fewer items, Romanian would normally express that more clearly in another way, for example:

• Mănânc mai puține feluri de mâncare seara.
• Mănânc mai puține lucruri seara.

So in your sentence, the most natural meaning is simply I eat less in the evening.

A rough English-friendly guide:

• mănânc ≈ muh-NUNK
• seara ≈ SYA-ra

A few pronunciation notes:

• ă is a short central vowel, like a relaxed uh
• â / î is a deeper central sound that English does not really have
• c at the end of mănânc is pronounced like k

So mănânc has sounds that may feel unusual at first, especially because it contains both ă and â.

Not in this sentence.

For general time expressions like in the evening, Romanian normally says:

• seara

not în seară.

Examples:

• Lucrez seara. = I work in the evening.
• Citesc seara. = I read in the evening.

However, în seara aceasta means this evening, where în is part of a different structure:

• În seara aceasta mănânc acasă. = This evening I’m eating at home.

So for the sentence you gave, seara is the correct natural choice.

By default, the present tense here usually suggests a habit or a general pattern:

• Eu mănânc mai puțin seara. = I eat less in the evening.

It sounds like something you generally do.

Romanian present tense can also describe what is happening now in the right context, but with seara this sentence most naturally sounds habitual.

You could, but it sounds more formal, heavier, and less natural for everyday speech.

Compare:

• Eu mănânc mai puțin seara. = natural, everyday Romanian
• Eu mănânc mai puțin în timpul serii. = more literal, more formal, less common

For normal conversation, seara is the best choice.