Если муравей идёт по столу, я убираю крошки со стола.

Because Russian often uses the present tense in both parts of a sentence to describe a general situation, habit, or what usually happens.

So this sentence does not have to mean one single event happening right now. It can mean something like:

• If / whenever an ant walks on the table, I remove the crumbs from the table.

For a one-time future situation, Russian would more naturally use perfective future forms, for example:

• Если муравей пойдёт по столу, я уберу крошки со стола.

It can definitely feel like when or whenever in this kind of sentence.

In English, if can sound hypothetical. In Russian, если + present tense often describes a regular reaction:

• Если муравей идёт по столу, я убираю крошки со стола
  = If / whenever an ant is walking on the table, I clear the crumbs off the table.

So the meaning is often broader than a single hypothetical if.

This is the classic идти vs ходить distinction.

• идти = going in one direction, a specific instance of movement
• ходить = going around habitually, repeatedly, or without a single directed path

So муравей идёт по столу suggests a specific ant moving across the table at that moment or in each such situation.

If you said муравей ходит по столу, it would sound more like:

• the ant goes around on the table regularly
• the ant wanders about on the table
• this is a repeated or characteristic action

Because по столу emphasizes movement across/along the surface of the table.

• по столу = along the table, over the table surface
• на столе = on the table, simply location

So:

• муравей идёт по столу = the ant is moving over the table
• муравей на столе = the ant is on the table

Both can be correct in different contexts, but по столу is better when the idea is specifically movement.

Because in this meaning, по takes the dative case.

So:

• стол = nominative
• по столу = dative after по

This use of по + dative is very common with movement over a surface:

• идти по дороге = go along the road
• ходить по комнате = walk around the room
• ползти по стене = crawl along the wall

Here убираю means I clean up, I remove, or I clear away.

The verb убирать can have several related meanings depending on context:

• убирать комнату = clean the room
• убирать вещи = put things away
• убирать крошки со стола = remove/clean crumbs off the table

So in this sentence, it is not just a general cleaning idea. It specifically means that the speaker clears the crumbs off the table.

Because the sentence describes a repeated, usual, or habitual action.

• убираю = imperfective, focuses on the process or repeated action
• уберу = perfective future, focuses on a single completed result

So:

• Если муравей идёт по столу, я убираю крошки со стола
  = this is what I do in such situations

But:

• Если муравей пойдёт по столу, я уберу крошки со стола
  = if that happens once in the future, I will remove the crumbs

Because крошки is the accusative plural form here, and крошки is a direct object.

The important point is that крошка is inanimate, so in the plural:

• nominative plural = крошки
• accusative plural = крошки

That is why the form does not change.

Compare with animate nouns, where accusative plural is different:

• я вижу студентов = I see the students

But:

• я вижу крошки = I see the crumbs

Russian often uses со instead of с when it makes pronunciation easier.

Here, с стола would be awkward because of the consonant cluster, so standard Russian prefers:

• со стола

This still means from/off the table.

Also, after с/со in this meaning, you need the genitive case:

• стол → стола

So:

• со стола = from the table / off the table

Because с/со meaning from/off takes the genitive case.

So the forms are:

• стол = nominative
• со стола = genitive after со

This is very common:

• с полки = from the shelf
• со стены = from the wall
• с окна = from the window

So in the sentence:

• убираю крошки со стола = I remove the crumbs from the table

Because each prepositional phrase has its own job:

• по столу = where the ant is moving
• со стола = where the crumbs are being removed from

Russian does not try to avoid repetition as strongly as English sometimes does. Repeating the noun is completely natural and clear here.

Yes. Russian word order is flexible, because cases show grammatical relationships.

For example, you could also say:

• Если муравей идёт по столу, я со стола убираю крошки.
• Я убираю крошки со стола, если муравей идёт по столу.

But the original version is the most neutral and natural.

The usual effect of changing word order is not changing the basic meaning, but changing emphasis or focus.

You know from context, not from an article.

Russian has no words equivalent to English a/an and the as regular articles. So муравей can mean:

• an ant
• the ant
• sometimes just ant in a general sense

In this sentence, English would often use an ant because the idea is general: if an ant walks on the table...

But in another context, the same Russian word could mean the ant if both speaker and listener already know which ant is meant.