Я вымыла палитру после урока, чтобы краски не засохли.

Because вымыла is the perfective past tense of вымыть, meaning washed completely / cleaned off.

Here the speaker is talking about a completed action: the palette got washed after the lesson.
If you said мыла, that would be the imperfective verb, which focuses more on the process: was washing / used to wash / washed without emphasizing completion.

So in this sentence, вымыла fits better because the result matters: the palette was cleaned so the paints would not dry out.

In the past tense, Russian verbs agree with gender and number.

• вымыл = masculine
• вымыла = feminine
• вымыло = neuter
• вымыли = plural

So Я вымыла tells us the speaker is female.
If the speaker were male, it would be Я вымыл палитру...

Because палитру is the accusative case, which is used for the direct object of the verb.

The basic dictionary form is:

• палитра = palette

But after a verb like вымыть (to wash), the thing being washed is the direct object, so палитра changes to палитру.

For feminine nouns ending in -а:

• nominative: палитра
• accusative: палитру

Because the preposition после (after) requires the genitive case.

The noun is:

• урок = lesson, class

In the genitive singular, it becomes:

• урока

So:

• после урока = after the lesson

This is a very common pattern in Russian:

• после работы = after work
• после фильма = after the film
• после обеда = after lunch

Чтобы introduces a purpose clause. Here it means:

• so that
• in order that
• sometimes just so

So:

• Я вымыла палитру после урока, чтобы краски не засохли.
• I washed the palette after the lesson so that the paints wouldn’t dry out.

It explains why the speaker washed the palette.

Here краски is the nominative plural, because it is the subject of не засохли.

Singular:

• краска = paint / a paint

Plural:

• краски = paints / paint(s)

In this sentence, the paints are the things that might dry out, so they are the grammatical subject of the subordinate clause:

• краски не засохли = the paints didn’t dry out / wouldn’t dry out

That is a very common Russian pattern.

After чтобы, Russian often uses a past-tense verb form to express something desired, intended, or prevented in relation to another action. In English, we often translate it with would, could, or an infinitive structure.

So:

• чтобы краски не засохли

literally looks like:

• so that the paints did not dry out

but in natural English it usually means:

• so that the paints wouldn’t dry out

Also, засохли is plural because it agrees with краски.

• сохнуть = to dry, to be drying
• засохнуть = to dry out, to become dried up

The prefix за- often adds the idea of reaching a result or state.
So засохнуть is a good choice for paints, because it means they become unusable by drying out.

That is why не засохли works well here: the speaker washed the palette to prevent that result.

Because both parts of the sentence focus on results:

• вымыла = she finished washing the palette
• засохли = the paints would end up dried out

The sentence is about one completed action done to prevent another completed result.

If you used imperfective forms, the meaning would shift toward process or general ongoing action rather than a specific completed event.

Yes. Russian word order is more flexible than English, although the original sentence is very natural.

For example, you could also say:

• После урока я вымыла палитру, чтобы краски не засохли.

This puts more emphasis on after the lesson.

Or:

• Чтобы краски не засохли, я вымыла палитру после урока.

This puts more emphasis on the purpose.

The original version is neutral and natural:

• Я вымыла палитру после урока, чтобы краски не засохли.

Yes, often it can.

Because вымыла already shows:

• past tense
• singular
• feminine

Russian speakers often omit Я if it is clear from context:

• Вымыла палитру после урока, чтобы краски не засохли.

This can sound perfectly natural in conversation or narration.
Including Я just makes the subject explicit.

It can mean either, depending on context.

• урок often means lesson
• in school or course contexts, it can also be understood as class

So после урока could be translated as:

• after the lesson
• after class

If this is about painting or art, after the lesson or after class are both possible natural translations.

Usually палитра is best translated as palette, especially in an art context.

A palette is the surface where a painter mixes paints.
Paint tray is possible in some contexts, but it sounds less standard here unless you specifically mean a tray-like container.

So for this sentence, palette is the most natural choice.