Learn Hardcore Portuguese (Portugal): Leva uma manta para o piquenique e escolhe um relvado sem formigas. - Take a blanket for the picnic and choose a lawn without ants.

It’s addressing the informal singular tu. The affirmative imperatives leva and escolhe are the forms used when speaking to one person you’re on familiar terms with (common in Portugal).

To a polite/formal singular você: Leve… e escolha…
To a group (vocês): Levem… e escolham…

How are the imperatives leva and escolhe formed? What about the negative?

Affirmative imperative with tu: take the tu present indicative and drop the final -s.
- tu levas → leva
- tu escolhes → escolhe
Negative imperative uses the present subjunctive:
- Não leves…
- Não escolhas…

Could I say this to more than one person?

Yes, use the vocês imperative:

Levem uma manta para o piquenique e escolham um relvado sem formigas.

Why levar and not trazer?

In Portuguese, levar is “to take (from here to there)” and trazer is “to bring (to here)”.

If you and the listener are not at the picnic location: Leva uma manta…
If you’re at (or speaking as the host at) the picnic location expecting the person to come: Traz uma manta para o piquenique.

Why is it para o piquenique with a definite article? Could I say para um piquenique or drop the article?

para o piquenique = for the (specific/planned) picnic. Portuguese uses definite articles more than English in these context-specific references.
para um piquenique = for a picnic (in general), which is also fine if you mean any picnic.
Dropping the article (para piquenique) is not idiomatic in European Portuguese.

Is para the best preposition here? Could I use a or por?

para is ideal for purpose/destination: “for the picnic.”
a/ao emphasizes motion “to” a place (e.g., “take it to the picnic”), and can be heard, but para o piquenique is more natural for the idea of taking something for use at the event.
por is not used here.

Can para o be contracted to pro/pra?

In European Portuguese, stick to para o in writing. In casual speech you may hear reductions like p’ra (especially with para a), but pro/pra is a Brazilian usage and not standard in Portugal.

What’s the nuance of relvado versus relva? What would Brazilians say?

relva = grass (the plant/cover).
relvado = a lawn/grass-covered area (the place). Here, um relvado is natural because you’re choosing a spot. In Brazil, gramado is the common word for a lawn: um gramado.

Why is it sem formigas and not sem as formigas?

sem formigas means “without ants” in general—i.e., choose a lawn that has no ants. sem as formigas would imply some specific, previously mentioned ants and sounds odd here.

How would I replace uma manta and um relvado with object pronouns?

With affirmative imperatives in European Portuguese, pronouns attach after the verb with a hyphen:

Leva-a (take it = the blanket; manta is feminine).
Escolhe-o (choose it = the lawn; relvado is masculine). In the negative, pronouns go before the verb:
Não a leves.
Não o escolhas.

Any pronunciation tips (European Portuguese)?

Leva: LEH-vuh (short, open e).
manta: MUN-tuh (nasal an).
piquenique: pee-kee-NEEK(uh) (final vowel very reduced).
e: like English “eh.”
escolhe: sh-KOL-yeh (initial es = “sh”; lh like the “lli” in “million”).
relvado: hehl-VAH-doo (guttural r; dark l).
sem: nasal, roughly “seng.”
formigas: fohr-MEE-gush (final s sounds like “sh” in Portugal).

Could I rephrase it and still sound natural?

Yes, for example:

Leva uma manta para o piquenique e escolhe um relvado sem quaisquer formigas. (a bit more emphatic)
Traz uma manta para o piquenique e escolhe um relvado sem formigas. (if “bring” fits the context)
Leva uma manta para o piquenique e escolhe uma zona de relva sem formigas. (using zona de relva)

Is the e just “and,” or could I use punctuation or sequencing words?

e is simply “and.” You could also write:

Leva… e depois escolhe… (and then choose…)
Leva…, depois escolhe… (comma plus depois is common in instructions)