Я положил картон под холст, чтобы не испачкать стол акварелью.

Положил is the masculine singular past tense of the perfective verb положить (to put, to place).

Why this form:

• Я = I
• The speaker is understood to be male because of положил
• If the speaker were female, it would be Я положила

Why положить instead of класть:

• положить is perfective: it presents the action as a completed whole
• класть is imperfective: it focuses more on process, repetition, or an uncompleted action

So Я положил картон... means something like I put / I placed the cardboard... as a completed action.

Картон is in the accusative case, because it is the direct object of положил.

In this sentence:

• положил что? → картон

For many inanimate masculine nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: картон
• accusative: картон

That is why there is no visible ending change.

Because под can take two different cases, depending on meaning:

• под + accusative = movement to a position under something
• под + instrumental = location under something

Here the sentence describes placing something under the canvas, so it involves movement toward that position.

That is why:

• под холст = under the canvas (motion, destination)

Compare:

• Я положил картон под холст. = I put the cardboard under the canvas.
• Картон лежит под холстом. = The cardboard is lying under the canvas.

Холст is in the accusative singular, because after под with motion, Russian uses the accusative.

Again, холст is an inanimate masculine noun, so:

• nominative singular: холст
• accusative singular: холст

So the form does not visibly change.

Чтобы introduces a purpose clause. It means so that, in order to, or so as to.

In this sentence:

• Я положил картон под холст, чтобы не испачкать стол акварелью.
• literally: I put cardboard under the canvas so that / in order not to dirty the table with watercolor.

So чтобы tells us the reason or goal for the action in the main clause.

Russian often uses чтобы + infinitive when the subject of both actions is the same.

Here, the same person:

• положил the cardboard
• wanted not to stain the table

So the infinitive is very natural:

• чтобы не испачкать стол

If the subject were different, Russian would often use a finite verb:

• Я положил картон под холст, чтобы стол не испачкался. = I put cardboard under the canvas so that the table wouldn’t get stained.

So the infinitive version keeps the focus on my purpose.

Испачкать is perfective and means to stain / to make dirty as a completed result.

After чтобы, Russian very often uses a perfective infinitive when talking about a goal or a result to be achieved or avoided.

Here the meaning is:

• I put the cardboard there in order not to end up staining the table

So не испачкать means not to stain in the sense of avoiding the completed bad result.

If you used the imperfective пачкать, it would sound more like not to be staining / not to dirty habitually, which is less natural here.

Because не directly negates the infinitive испачкать.

So:

• чтобы не испачкать стол = so as not to stain the table

This is the normal placement in Russian:

• не goes before the verb it negates

You can think of the whole phrase as one unit of purpose:

• in order not to stain the table

Because стол is the direct object of испачкать.

Ask:

• испачкать что? → стол

Since стол is an inanimate masculine noun, its accusative singular is the same as its nominative:

• nominative: стол
• accusative: стол

So again, the form looks unchanged.

Акварелью is the instrumental singular of акварель.

Here the instrumental expresses the means, material, or substance by which something gets stained.

So:

• испачкать стол акварелью = to stain the table with watercolor

This is a very common use of the instrumental in Russian:

• писать карандашом = to write with a pencil
• резать ножом = to cut with a knife
• испачкать краской = to stain with paint

So акварелью answers something like:

• чем? → with what? → with watercolor

It can mean both, depending on context:

• watercolor paint
• watercolor painting / watercolor technique

In this sentence, because of испачкать стол акварелью, it clearly means the paint/material: with watercolor paint.

If necessary, Russian could be more explicit with:

• акварельной краской = watercolor paint

But акварелью is natural and idiomatic here.

Russian has no articles like English a/an and the.

So:

• картон can mean cardboard, a piece of cardboard, or the cardboard, depending on context
• холст can mean canvas or the canvas
• стол can mean table or the table

The listener understands definiteness from context, not from an article.

That is why the Russian sentence can sound more compact than the English translation.

The given word order is natural and neutral:

• Я положил картон под холст, чтобы не испачкать стол акварелью.

Russian word order is more flexible than English because case endings show grammatical roles. So other orders are possible, but they change emphasis.

For example:

• Под холст я положил картон, чтобы не испачкать стол акварелью.
  • emphasizes under the canvas
• Чтобы не испачкать стол акварелью, я положил картон под холст.
  • emphasizes the purpose

So the original sentence is a straightforward, unmarked way to say it.