Antes de salir, ponte repelente, que en el bosque hay muchos mosquitos y alguna mosca pesada.

After a preposition like de, Spanish uses the infinitive, not a conjugated verb. So:

• antes de salir = before leaving / before you leave

Also, this structure is especially common when the person doing both actions is the same person: the person who will leave is also the person being told to put on repellent.

If the subject changes, Spanish often uses antes de que + subjunctive instead:

• Antes de salir, ponte repelente.
• Antes de que los niños salgan, ponles repelente.

So antes de sales is not possible here.

Ponte is the affirmative tú command of ponerse.

It breaks down like this:

• infinitive: ponerse
• command for tú: pon
• reflexive pronoun: te
• combined: ponte

So ponte repelente means put on some repellent.

With affirmative commands, pronouns are attached to the end:

• ponte
• siéntate
• lávate

But with negative commands, the pronoun goes before the verb:

• no te pongas repelente (or more naturally, no te pongas eso in another context)

Spanish very often uses ponerse for putting something on yourself, including things like:

• ponerse crema
• ponerse perfume
• ponerse protector solar
• ponerse repelente

To an English speaker, put on repellent may sound a little odd at first, but in Spanish it is completely natural.

You could also hear echarse repelente, which is also common and means something like put on / apply repellent.

So ponte repelente is idiomatic Spanish.

Without an article, repelente is being used in a general sense, like put on repellent.

• Ponte repelente = put on some repellent / put on repellent

If you say ponte el repelente, it sounds more specific:

• the repellent we have here
• the repellent we already mentioned
• that particular product

So both are possible, but the version without the article is very natural when giving a general instruction.

Yes, very often.

In context, repelente by itself usually means insect repellent. Spanish often leaves out de insectos when it is obvious.

So:

• repelente
• repelente de insectos

can both work, but the shorter version is very common in everyday speech.

Here que means something like because / since.

So:

• Ponte repelente, que en el bosque hay muchos mosquitos...

means:

• Put on repellent, because/since there are lots of mosquitoes in the woods...

This use of que is very common in spoken Spanish, especially after commands or suggestions. It sounds natural and conversational.

Compare:

• Ponte repelente, que hay mosquitos. = Put on repellent, because there are mosquitoes.
• Ponte repelente porque hay mosquitos. = same basic meaning, slightly more explicit

Hay is the impersonal present form of haber meaning there is / there are.

It is used for both singular and plural nouns:

• hay un mosquito = there is one mosquito
• hay muchos mosquitos = there are many mosquitoes

Unlike English, Spanish does not change the verb here for singular vs. plural. It stays hay.

Alguna in the singular often means some odd / some occasional / the occasional.

So:

• alguna mosca pesada

suggests something like:

• the occasional annoying fly
• a pesky fly or two
• some annoying fly here and there

It does not focus on a large number. It gives a lighter, less exact idea than algunas moscas pesadas, which would sound more like some annoying flies as a group.

So the contrast is natural:

• muchos mosquitos = lots of mosquitoes
• alguna mosca pesada = and the occasional annoying fly

Literally, pesado / pesada can mean heavy, but here it means annoying, pesky, or a nuisance.

So:

• una mosca pesada = an annoying fly / a pesky fly

This is a very common figurative meaning in Spanish, especially in Spain:

• Qué pesado eres. = You’re so annoying.
• Un vecino pesado. = An annoying neighbor.

Here pesada agrees with mosca, which is feminine singular.

En el bosque means in the woods / in the forest.

• en = in
• el bosque = the forest / the woods

Spanish uses the definite article here very naturally, even where English might sometimes just say in the woods without thinking much about the article.

So en el bosque is exactly what you would expect in Spanish.

Yes, it sounds very natural in Spain.

A few things that make it feel especially Spain-friendly are:

• ponte as the informal singular command
• que used to mean because/since in a conversational way
• mosca pesada, which sounds very natural in Peninsular Spanish

None of this would be strange elsewhere, but the tone is very everyday, casual Spanish as spoken in Spain.