Learn Hardcore Russian: В детстве я мечтал стать художником и много рисовал карандашами. - In childhood I dreamed of becoming an artist and drew a lot with pencils.

детстве is the prepositional case of детство (childhood).
The preposition в
- prepositional is commonly used to talk about time periods when something happened:
  - в детстве – in childhood
  - в молодости – in youth
  - в прошлом году – last year

If you said в детство (accusative), it would mean into childhood (direction, movement), which doesn’t fit here. For a time reference, you need в детстве.

What tense and form is мечтал, and how would it change for a woman or for plural?

Мечтал is:

past tense
imperfective aspect (from the verb мечтать)
masculine singular form

So я мечтал implies that the speaker is male.

Other forms:

Feminine singular: я мечтала
Neuter singular (for neuter subjects, not people): оно мечтало
Plural (any mixed or all-male group): мы мечтали, они мечтали

So a woman would naturally say:

В детстве я мечтала стать художником…

Why is мечтал (imperfective) used here, not a perfective form?

Имperfective is used because the sentence describes a long‑lasting or repeated state in the past – something that was generally true during childhood.

мечтал – I used to dream / I would dream / I dreamed (habitually or over a period)
A perfective form would describe one completed act of dreaming, which doesn’t fit the idea of a general childhood dream.

Similarly, рисовал is also imperfective, showing repeated or ongoing action:

я много рисовал – I drew a lot / I used to draw a lot (habitually).

Why do we say мечтал стать художником and not мечтал быть художником?

With мечтать (to dream), Russian almost always uses стать + instrumental case to express dreaming of becoming something:

мечтать стать врачом / музыкантом / художником

Стать is perfective and focuses on the result – the moment of becoming that thing. When you dream of a future profession or role, you usually imagine that result.

Быть can sound possible in some contexts, but:

мечтал быть художником is unusual and, if used, tends to focus more on already being in that role, not the process of becoming it.
The natural, idiomatic choice is мечтать стать кем‑то.

Why is художником in the instrumental case? Why not just художник?

After the verb стать (to become), Russian uses the instrumental case to show what someone becomes:

стать врачом – to become a doctor
стать учителем – to become a teacher
стать художником – to become an artist

So художник (nominative) changes to художником (instrumental, singular, masculine):

Nominative: художник
Instrumental: художником

The instrumental case is standard after verbs like:

быть – быть художником (to be an artist)
стать – стать художником (to become an artist)
работать – работать художником (to work as an artist)

Why is я not repeated before много рисовал? Is it optional?

Russian often omits the repeated subject when it stays the same within a sentence, especially after и (and):

В детстве я мечтал стать художником и много рисовал карандашами.

The subject я clearly applies to both verbs мечтал and рисовал, so repeating it is not necessary.

You could say:

В детстве я мечтал стать художником и я много рисовал карандашами.

This is grammatically correct, but it sounds heavier and is usually only used for emphasis or in very deliberate speech. The version without the second я is more natural.

Why is карандашами in the instrumental plural? Could we say карандашом instead?

Карандашами is the instrumental plural of карандаш (pencil):

Nominative plural: карандаши
Instrumental plural: карандашами

The instrumental case without a preposition often expresses the tool or means used to do something:

писать ручкой – to write with a pen
резать ножом – to cut with a knife
рисовать карандашами – to draw with pencils

Difference between карандашом and карандашами:

рисовать карандашом – with one pencil / in pencil (as a technique, singular)
рисовать карандашами – with pencils in general, or with several pencils (plural), which is natural when you think of a set of colored pencils, many different ones, etc.

In this sentence, карандашами suggests drawing a lot and using pencils in general, possibly many different ones.

Is there any difference between много рисовал карандашами and рисовал много карандашами?

Both are possible, but the usual, neutral word order is:

много рисовал карандашами – drew a lot with pencils

Word order in Russian is quite flexible, but it affects emphasis:

много рисовал карандашами – focuses on how much he drew (a lot).
рисовал много карандашами – can sound a bit unusual; the adverb много is more natural closer to the verb.

You could also hear:

рисовал карандашами много – this puts extra emphasis on много, often in contrast to something else (he drew with pencils a lot, but rarely painted with paints, etc.).

In most contexts, много рисовал карандашами is the best choice.

Why are there no words like a / an / the before художником or карандашами?

Russian does not have articles (no direct equivalents of a, an, the).
Whether something is a(n) or the is understood from context, not from a separate word.

So:

стать художником can mean to become an artist or to become the artist, depending on context.
карандашами can mean with pencils, with the pencils, etc., again depending on context.

Learners just have to get used to the fact that Russian expresses definiteness/indefiniteness through context and word order, not through articles.

Is the whole sentence in the past tense? What about стать – isn’t that future?

The sentence as a whole describes the past, but стать here is an infinitive, not a finite verb:

мечтал – past tense, imperfective (he dreamed / used to dream)
стать – infinitive (to become), part of what he was dreaming about
рисовал – past tense, imperfective (he drew / used to draw)

So the structure is:

мечтал (о чём?) стать художником – he dreamed of becoming an artist

The time of стать (becoming) is understood as future relative to that past (he wanted to become an artist later), but grammatically we only mark past on мечтал, not on стать. The infinitive itself doesn’t carry tense.