Тебе пора бы убрать со стола лишние вещи, если ты хочешь спокойно рисовать.

Because пора uses the person in the dative case.

The pattern is:

кому + пора + infinitive
= it’s time for someone to do something

So:

• тебе пора = it’s time for you
• мне пора = it’s time for me
• нам пора = it’s time for us

Russian is not saying you are time or using you as the subject. It is more like to you, it is time to...

Пора means it’s time or the time has come.

In Russian, пора is often used as a predicate word, not like a normal noun in this structure. It commonly appears with an infinitive:

• Пора идти. = It’s time to go.
• Тебе пора спать. = It’s time for you to sleep.

So in your sentence, тебе пора убрать... means it’s time for you to clear away...

Бы adds nuance. Here it makes the sentence sound less like a neutral statement and more like:

• you really should...
• it would be about time to...
• you ought to...

So тебе пора бы убрать... has a mild sense of suggestion, pressure, or even gentle reproach.

Compare:

• Тебе пора убрать со стола... = straightforward: it’s time for you to clear the table
• Тебе пора бы убрать со стола... = softer or slightly more pointed: you really ought to clear the table by now

This is a very natural colloquial pattern.

Because убрать is perfective, and the sentence is talking about a single completed action: clearing the extra things away.

• убирать = imperfective, process / repeated action / general activity
• убрать = perfective, complete the action

Here the idea is not be in the process of tidying but get it done.

So:

• пора убрать = it’s time to clear it away
• пора убирать would sound more like it’s time to start tidying / time to be tidying up, depending on context

There are two separate things here:

1. со is a variant of с
2. стола is in the genitive case

Here с/со means from / off when something is removed from a surface.

So:

• убрать со стола = remove from the table / clear off the table

Why со instead of с?
Russian often uses со instead of с to make pronunciation easier, especially before awkward consonant clusters. Since стола begins with ст-, со стола is easier to say than с стола.

Both may sometimes be heard, but со стола is the standard, natural form.

Because the preposition с/со takes the genitive when it means from.

Here the meaning is movement/removal away from the table, so Russian uses:

с/со + genitive

Examples:

• со стола = from the table
• с полки = from the shelf
• с окна = from the windowsill/window area

So стол becomes стола in the genitive singular.

Лишние вещи means extra things, unnecessary things, or things that shouldn’t be there.

• лишний = extra, superfluous, unnecessary
• вещи = things, items, belongings

In this sentence, it means the objects on the table that are getting in the way of drawing.

So it is not just random things, but specifically things that are unnecessary for what you’re doing.

It is in the accusative plural, because it is the direct object of убрать.

You are removing what?
→ лишние вещи

For inanimate plural nouns, the accusative usually looks the same as the nominative:

• nominative: лишние вещи
• accusative: лишние вещи

So the form does not change here, even though the grammatical function is different.

Because спокойно is an adverb, and it describes how you draw.

• спокойный = calm (adjective, describing a noun)
• спокойно = calmly / peacefully (adverb, describing a verb)

Here it modifies рисовать:

• рисовать спокойно = to draw calmly / peacefully
• спокойно рисовать = same idea, just different word order

Russian uses the adverb because it is describing the action, not a person or object.

Because after хотеть (to want), Russian normally uses an infinitive for the action wanted.

So:

• хочешь рисовать = you want to draw
• хочу читать = I want to read
• они хотят поехать = they want to go

This is similar to English want to do something.

Yes, it could be omitted:

• если хочешь спокойно рисовать is also perfectly natural

Russian often leaves out subject pronouns when they are clear from the verb ending. Since хочешь already means you want, ты is not grammatically necessary.

Including ты can add a little emphasis, clarity, or conversational directness.

So:

• если хочешь... = natural, slightly lighter
• если ты хочешь... = a bit more explicit or emphatic

Because если ты хочешь спокойно рисовать is a subordinate clause introduced by если (if).

Russian normally separates subordinate clauses with a comma:

• Я пойду, если будет время.
• Позвони, если сможешь.
• Тебе пора бы убрать со стола лишние вещи, если ты хочешь спокойно рисовать.

So the comma is required here.

Yes. Russian word order is flexible, and several versions would be possible.

For example:

• Тебе пора бы убрать лишние вещи со стола...
• Если ты хочешь спокойно рисовать, тебе пора бы убрать со стола лишние вещи.

The original order sounds natural and conversational. Putting со стола before лишние вещи is fine and often feels smooth after убрать.

Changing the order can shift emphasis a little, but not the basic meaning.