Tant si arriba avui com si arriba demà, recull-lo.

It means whether ... or ... or regardless of whether ... or ....

So:

• Tant si arriba avui com si arriba demà, recull-lo.
• = Whether he/it arrives today or tomorrow, pick him/it up.

This is a very common Catalan pattern for saying that the result in the main clause stays the same in either case.

Not in this sentence.

By itself, com si often means as if:

• Parla com si ho sabés = He speaks as if he knew it

But in the fixed structure tant si ... com si ..., it does not mean as if. Here it is just part of the correlative expression meaning whether ... or ....

So you should read the whole pattern together:

• tant si X com si Y = whether X or Y

Because si without an accent means if / whether.

Catalan also has sí with an accent, but that means yes.

So:

• si = if, whether
• sí = yes

In this sentence, it is clearly the conjunction si, so there is no accent.

Because Catalan often uses the present tense to talk about future events, especially in subordinate clauses like this one.

So arriba avui / arriba demà can naturally mean:

• arrives today / tomorrow
• is arriving today / tomorrow
• will arrive today / tomorrow, depending on context

Also, after si, Catalan normally does not use the future in the way English learners might expect. So a learner might imagine something like si arribarà, but that is not the normal choice here.

Using arriba is the natural Catalan wording.

Because each alternative is presented as its own clause:

• si arriba avui
• com si arriba demà

This repetition is normal and clear in Catalan. It matches the structure whether he arrives today or whether he arrives tomorrow.

You sometimes can avoid repeating the full verb in other ways, but in this pattern, repeating it is the most straightforward option.

Recull-lo is made of two parts:

• recull = the tu imperative of recollir
• -lo = a direct-object pronoun meaning him or it

So grammatically it is:

• recull
  • lo
• pick up / collect
  • him/it

This makes recull-lo an affirmative command to one person:

• Pick him/it up
• Collect him/it

Because in Catalan, affirmative imperatives normally take object pronouns after the verb. This is called enclisis.

So instead of putting the pronoun before the verb, Catalan attaches it:

• recull-lo
• dóna'm
• mira'l

The spelling uses a hyphen or an apostrophe, depending on the form and the surrounding sounds.

Here the correct written form is:

• recull-lo

not a separate lo.

It can mean either one.

-lo is the masculine singular direct-object pronoun, so it can refer to:

• him, if the referent is a male person
• it, if the referent is a masculine noun or something understood from context

You need context to know which one is intended.

So recull-lo could mean:

• pick him up
• pick it up
• collect him
• collect it

depending on what was mentioned earlier.

Yes. It is the singular informal command, used with tu.

So the speaker is telling one person informally to do something.

A formal singular command would use a different verb form, for example:

• reculli'l

So recull-lo specifically tells you that the sentence is addressing you in the tu form.

Because the sentence begins with a subordinate structure:

• Tant si arriba avui com si arriba demà

and then gives the main clause:

• recull-lo

The comma helps separate the whether ... or ... part from the main instruction. This is normal and helpful punctuation in Catalan, just as it often is in English with an introductory clause.

So the comma marks the boundary between:

• the condition/alternative frame
• the main command