Я хочу дорисовать небо и потом докрасить деревья.

In Russian, after хотеть (to want), the next verb is usually put in the infinitive, just like in English:

• Я хочу дорисовать... = I want to finish drawing...
• Я хочу докрасить... = I want to finish painting...

So the pattern is:

• я хочу + infinitive
• ты хочешь + infinitive
• он/она хочет + infinitive

This is very common and straightforward.

The verbs дорисовать and докрасить have the prefix до-, which often adds the idea of finishing something that was already started or doing something up to the end.

So:

• рисовать = to draw
• дорисовать = to finish drawing, to complete a drawing

and

• красить = to paint, to color
• докрасить = to finish painting, to paint the rest

This sentence suggests that the speaker has already started the picture and now wants to complete some remaining parts:

• finish drawing the sky
• then finish painting the trees

So до- is important here because it gives the sense of completion.

Yes. Both дорисовать and докрасить are perfective verbs.

In Russian, perfective verbs usually focus on a completed result. That fits this sentence well, because the speaker wants to finish the sky and the trees, not just work on them in a general ongoing way.

Their imperfective partners would be:

• дорисовывать = to be finishing / to finish repeatedly
• докрашивать = to be finishing painting / to finish repeatedly

In this sentence, the perfective infinitives are used because after хочу the speaker is talking about a goal: completing the actions.

Небо here is in the accusative case, because it is the direct object of дорисовать.

The speaker wants to finish drawing what?
→ небо (the sky)

For many neuter nouns in Russian, the accusative singular is the same as the nominative singular:

• nominative: небо
• accusative: небо

So even though the form does not change, the case function is still accusative.

Деревья is also in the accusative case, because it is the direct object of докрасить.

The speaker wants to finish painting what?
→ деревья (the trees)

The dictionary form is:

• singular: дерево = tree
• plural: деревья = trees

Because деревья is an inanimate plural noun, its accusative plural is the same as its nominative plural:

• nominative plural: деревья
• accusative plural: деревья

So the form stays the same.

The difference comes from the base verbs:

• рисовать = to draw
• красить = to paint / color

So:

• дорисовать небо means to finish drawing the sky, probably adding lines, shapes, or the remaining part of the image
• докрасить деревья means to finish painting/coloring the trees, probably adding the remaining color

This suggests the sentence is about making a picture: first complete the drawing of the sky, then complete the coloring of the trees.

Потом means then, after that, or later.

In this sentence:

• Я хочу дорисовать небо и потом докрасить деревья.
• I want to finish drawing the sky and then finish painting the trees.

It shows the order of actions:

1. finish the sky
2. then finish the trees

Russian word order is flexible, but here потом naturally goes before the second verb to show sequence.

И means and. Here it connects two infinitive phrases:

• дорисовать небо
• докрасить деревья

So the structure is:

• I want to do X and then do Y

That is why Russian uses:

• ... и потом ...

This is very natural and means something like ... and then ...

Yes, you could say:

• Я хочу дорисовать небо и докрасить деревья.

That would mean I want to finish drawing the sky and finish painting the trees.

But without потом, the sense of sequence is weaker. It just lists two things the speaker wants to do. With потом, it sounds more like a plan in order:

• first the sky
• then the trees

Russian does not have articles like the or a/an.

So:

• небо can mean sky or the sky
• деревья can mean trees or the trees

The exact meaning depends on context. In this sentence, English naturally uses the sky and the trees, but Russian does not need separate words for that.

Russian often drops subject pronouns when the verb ending already shows who the subject is.

So both of these are possible:

• Я хочу дорисовать небо и потом докрасить деревья.
• Хочу дорисовать небо и потом докрасить деревья.

Both mean the same thing. The version with Я is a little more explicit and can add slight emphasis or clarity. Including Я is completely normal.

Yes, very strongly. The prefixes до- in дорисовать and докрасить suggest that the work is already in progress and now needs to be completed.

So the sentence does not just mean:

• I want to draw the sky and paint the trees

It more specifically means:

• I want to finish drawing the sky and then finish painting the trees

That idea of completion of something already begun is one of the key things a learner should notice here.