Learn Hardcore Russian: Пожалуйста, не ешь много конфет вечером. - Please don’t eat a lot of candies in the evening.

The underlying personal form is ты ешь = you eat (informal singular).
To make a negative command to one person (informal), Russian uses не + 2nd person singular:
- (Ты) не ешь. = Don’t eat.

In spoken Russian, the pronoun ты is usually dropped because the verb ending already shows who is being addressed. So не ешь on its own clearly means (you, singular informal) don’t eat.

Is this sentence formal or informal? How would I say it politely to вы?

The sentence is informal, because ешь talks to ты (one person you are on familiar terms with: a child, friend, family member, etc.).

To say this to вы (formal or plural you), change the verb form:

Пожалуйста, не ешьте много конфет вечером.

Ешьте is the 2nd person plural imperative (for вы).
This can mean either:

talking politely to one person (Вы)
or talking to several people at once.

Why is it много конфет and not много конфеты or много конфетов?

After words of quantity like много (a lot, many), мало (few, little), сколько (how many/much), Russian uses the genitive plural for countable nouns.

Dictionary (nominative singular): конфета (a candy)
Genitive plural: конфет

So the grammatically correct pattern is:

много + [genitive plural] → много конфет

Forms like много конфеты or много конфетов are simply incorrect for standard Russian in this meaning.

What grammatical case is конфет, and why is it used here?

Конфет is in the genitive plural case.

Reasons:

The word много (a lot/many) requires the genitive case for the noun it quantifies.
Since we are talking about more than one candy, it must be plural.

So:

одна конфета – one candy (nom. sg.)
нет конфет – no candies (gen. pl.)
много конфет – many/a lot of candies (gen. pl.)

This is a regular and very common rule in Russian after quantity words like много, мало, сколько, несколько, сколько-нибудь.

Why is вечером used to mean “in the evening”? What case is it, and could I just say вечер?

Вечером is the instrumental singular of вечер (evening), but in this form it functions almost like an adverb of time, meaning:

вечером = in the evening / during the evening

You cannot use bare вечер here; не ешь много конфет вечер is ungrammatical.

Some common time expressions in this pattern:

утром – in the morning
днём – in the daytime
вечером – in the evening
ночью – at night

You could also say:

по вечерам – in the evenings (habitually, on evenings in general)

Can the word order be changed? For example, can I say Пожалуйста, вечером не ешь много конфет?

Yes, Russian word order is relatively flexible. All of these are grammatically correct:

Пожалуйста, не ешь много конфет вечером.
Пожалуйста, вечером не ешь много конфет.
Пожалуйста, не ешь вечером много конфет.

The basic meaning stays the same. Differences are mostly about focus:

Putting вечером earlier (e.g. Вечером не ешь…) slightly emphasizes the time (“In the evening, don’t eat a lot of candy”).
Putting много конфет earlier might stress how much is being eaten.

For a learner, the original word order is natural and neutral.

How does пожалуйста work here? Does its position (beginning vs end) change the tone?

Пожалуйста literally means please, and it softens a command or request.

Typical positions:

At the beginning:
- Пожалуйста, не ешь много конфет вечером.
  Sounds polite and a bit more formal.
At the end:
- Не ешь много конфет вечером, пожалуйста.
  Sounds natural and often a bit more emotional or insistent, especially in speech.

Both are polite; the difference is subtle. In both cases, it’s clearly a request rather than a blunt order.

How is a negative command formed in Russian, like не ешь?

A negative imperative (negative command) in Russian is formed by:

не + imperative (or 2nd person form)

For есть:

(ты) ешь – you eat
(ты) не ешь – don’t eat
(вы) ешьте – you (pl./formal) eat
(вы) не ешьте – don’t eat (plural or polite)

In this sentence:

не ешь directly means don’t eat (informal singular).

Could I use the perfective verb съесть instead of есть? What would be the difference between не ешь and не съешь?

Yes, есть has a perfective partner съесть.

есть (imperfective) – focuses on the process / repeated action:
- Не ешь много конфет вечером.
  Don’t (in general) eat a lot of candy in the evenings / Don’t be eating a lot of candy in the evening.
съесть (perfective) – focuses on the completed result:
- Не съешь много конфет вечером.
  This usually sounds odd as a warning; more likely, it might be used in a very specific context like a prediction:
  Ты не съешь много конфет вечером. – You won’t manage to eat many candies in the evening.

For a general prohibition or advice, Russian almost always uses the imperfective: Не ешь много конфет вечером.

Is есть here the same word as есть meaning “there is / there are”? How do Russians tell them apart?

They are homonyms in spelling but different in meaning and usually differ in stress and context:

Есть (to eat) – verb of eating, very common.
- In present tense: я ем, ты ешь, он ест…
Есть (there is / there are / exists) – a form of the verb быть, often optional in modern Russian:
- У меня есть время. – I have time (literally: at me there is time).

In Пожалуйста, не ешь много конфет вечером, context and the form ешь clearly show that this is the verb “to eat”, not the “there is” verb.

How would I say “Don’t eat too many candies in the evening” or “Don’t eat so many candies in the evening”?

You can modify много:

Не ешь слишком много конфет вечером.
– Don’t eat too many candies in the evening.
(слишком = too, overly)
Не ешь так много конфет вечером.
– Don’t eat so many candies in the evening.

Both are natural and keep the same structure as the original sentence.

How do you pronounce the key words in this sentence?

Approximate pronunciation (stressed syllables in caps):

Пожалуйста – pa-ZHA-lus-ta
не – nye
ешь – yesh (soft “sh”, like “sh” with the tongue closer to the front)
много – MNO-ga (the mn cluster is pronounced together: “mnó-ga”)
конфет – kan-FYET (the е after ф makes it фьe, softening the consonant a bit)
вечером – VYE-che-ram (first syllable stressed; вье is like “vye” in “yes” with a v in front)

Put together:
Пожалуйста, не ешь много конфет вечером.
= pa-ZHA-lus-ta, nye YESH MNO-ga kan-FYET VYE-che-ram.