Se vires uma migalha na mesa, limpa-a.

Because after se meaning if, Portuguese often uses the future subjunctive when talking about a possible future situation.

So:

• Se vires = if you see
• vês = you see in the present indicative

In English, we usually just use the present after if: If you see...
But Portuguese often prefers the future subjunctive in this kind of sentence:

• Se vires... = if you see...
• Quando fores... = when you go...
• Se tiveres tempo... = if you have time...

Here, vires is the tu form of the future subjunctive of ver.

The sentence is addressing one person informally, using tu.

You can tell from the verb forms:

• vires = tu
• limpa = command to tu

So this is the equivalent of speaking to one person you know well, such as a friend, child, sibling, or someone you would normally address as tu in European Portuguese.

A more formal version, using você, would be:

• Se vir uma migalha na mesa, limpe-a.

Because limpa here is the affirmative imperative form of limpar for tu.

With regular -ar verbs, the tu affirmative imperative usually looks like the present indicative tu form minus the final -s:

• tu limpas → limpa!

So:

• limpa = clean!

This is a command or instruction, not a statement.

Compare:

• Tu limpas a mesa. = You clean the table.
• Limpa a mesa. = Clean the table.

Because the object pronoun is attached to the end of the verb in an affirmative command. This is called enclisis.

So:

• limpa-a = clean it

In European Portuguese, this is very normal after affirmative imperatives:

• Faz isso. = Do that.
• Diz-me. = Tell me.
• Limpa-a. = Clean it.

The hyphen shows that the pronoun belongs to the verb.

It refers back to uma migalha.

Since migalha is a feminine singular noun, the direct object pronoun is a:

• uma migalha → a = it

So instead of repeating the noun:

• Se vires uma migalha na mesa, limpa a migalha.

Portuguese uses the pronoun:

• Se vires uma migalha na mesa, limpa-a.

This sounds much more natural.

Because migalha is feminine singular.

Portuguese direct object pronouns agree with the gender and number of the noun they replace:

• o = masculine singular
• a = feminine singular
• os = masculine plural
• as = feminine plural

Examples:

• o livro → Lê-o. = Read it.
• a migalha → Limpa-a. = Clean it.

Since migalha is feminine, a is the correct pronoun.

Because na is the contraction of em + a:

• em
  • a mesa → na mesa

Portuguese uses these contractions very regularly:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

Although em often means in, with locations like surfaces it can also correspond to English on:

• na mesa = on the table

So na mesa is the normal phrase here.

Migalha means crumb, usually a very small piece of bread, cake, biscuit, or other food.

It is a common everyday noun, and it is feminine:

• a migalha
• uma migalha
• as migalhas

So in this sentence, it simply means a crumb left on the table.

Yes. In a more formal or less familiar style, you could say:

• Se vir uma migalha na mesa, limpe-a.

Changes:

• vires → vir
• limpa → limpe

That is because the sentence now uses the verb forms associated with você.

So:

• Se vires..., limpa-a. = informal singular (tu)
• Se vir..., limpe-a. = formal singular (você)

The lh in Portuguese is pronounced like the lli in million for many English speakers, though not exactly the same.

So migalha sounds roughly like:

• mi-GA-lya

Very roughly, in European Portuguese pronunciation, the sentence might sound something like:

• Se vee-resh oo-ma mi-GA-lya na ME-zah, LEEM-pa-a.

A few useful pronunciation points:

• vires: the ending may sound softer in European Portuguese than you might expect from spelling
• lh in migalha is a special sound, not a normal l
• limpa-a keeps both parts: limpa + a, though in fast speech they flow together

Yes, it fits European Portuguese very well.

What makes it especially typical is:

• use of tu forms: vires, limpa
• use of the attached object pronoun in an affirmative command: limpa-a

These features are completely standard in Portugal.

A Brazilian Portuguese speaker would understand it, but everyday Brazilian speech often handles pronouns differently and may prefer different patterns in casual language. Since you are learning Portuguese from Portugal, this sentence is a very good model of standard European Portuguese usage.