Si fa fred, em poso els guants abans de sortir de casa.

Because Catalan uses the verb fer for many weather expressions.

• fa fred = it is cold / it’s cold
• fa calor = it is hot
• fa vent = it is windy

So fer here does not mean to make in the usual sense; it is just the normal idiomatic verb for weather conditions.

Using és fred would usually describe something as cold by nature, not the weather in general.

Em poso comes from the verb posar-se, which often means to put on clothing.

• poso = I put
• em poso = I put on myself / I put on

The em is a reflexive pronoun, meaning the action is being done to yourself. With clothes, Catalan often uses the reflexive form:

• Em poso l’abric = I put on my coat
• Es posa les sabates = He/She puts on his/her shoes

So in this sentence, em poso els guants is the natural way to say I put on my gloves / I put on gloves.

In normal finite verb forms, Catalan object and reflexive pronouns usually go before the verb.

• em poso
• et poses
• es posa

This is standard pronoun placement in Catalan.

Compare:

• Em poso els guants.
• Me’n vaig.
• Li ho dono.

Pronouns can go after the verb in some structures, such as infinitives, gerunds, and positive commands:

• posar-me
• posant-me
• posa’t els guants

But here, with a present tense verb, em goes before poso.

Catalan often uses the definite article with clothing and body-related items where English often uses a possessive or no article.

So Catalan prefers:

• em poso els guants
• em rento les mans
• es treu la jaqueta

English would usually say:

• I put on my gloves
• I wash my hands
• He takes off his jacket

The article els does not necessarily mean the in the strong English sense here. It is simply the normal Catalan structure.

After abans, Catalan normally uses de before an infinitive.

• abans de sortir = before going out
• abans de menjar = before eating
• abans de dormir = before sleeping

So the pattern is:

abans de + infinitive

If a full clause follows, then you can get a different structure, such as:

• abans que surti = before he/she goes out

But in your sentence, the verb is an infinitive, so de is required.

Casa often behaves a bit like home in English. In many common expressions, Catalan uses casa without an article when it means home rather than a specific physical house.

So:

• sortir de casa = leave home / go out from home
• ser a casa = be at home
• arribar a casa = arrive home

If you say de la casa, that usually sounds more like you are talking about a specific building:

• Va sortir de la casa blanca del final del carrer. = He/She came out of the white house at the end of the street.

So de casa is the idiomatic form here.

Because this is a real or general condition, and Catalan uses the indicative after si in this kind of sentence.

• Si fa fred, em poso els guants.
• Si plou, em quedo a casa.

This is similar to English If it’s cold, I put on gloves or If it rains, I stay home.

Catalan does not use the subjunctive after si in ordinary if-clauses like this. That is a very common thing learners need to remember.

By default, it sounds like a general habit or usual action:

• If it’s cold, I put on gloves before leaving home.

Both verbs are in the present tense, which often expresses routine or general truth in Catalan.

If you wanted a specific past event, you would change the tense:

• Si feia fred, em posava els guants abans de sortir de casa.
• Si ha fet fred, m’he posat els guants...
  though the exact tense depends on context.

So the original sentence most naturally sounds habitual.

Yes, but it means something slightly different.

• Si fa fred = if the weather is cold
• Si tinc fred = if I feel cold

So:

• Si fa fred, em poso els guants. focuses on the outside temperature.
• Si tinc fred, em poso els guants. focuses on your personal sensation.

Sometimes both work, but they are not exactly the same.

It is standard and helpful to use a comma when the conditional clause comes first:

• Si fa fred, em poso els guants abans de sortir de casa.

The comma makes the structure clearer:

1. condition
2. main clause

In informal writing, some people may omit it, but using it is a good habit.

No. Posar and posar-se are very common verbs with several meanings.

Some examples:

• posar un llibre a la taula = put a book on the table
• posar música = put on music
• posar-se l’abric = put on your coat
• posar-se content = become happy
• posar-se malalt = become ill

So in your sentence it means to put on, but the verb is much broader than that.