No me pongo el jersey verde a no ser que refresque por la tarde.

Because the verb here is ponerse, which is the reflexive form of poner.

• poner = to put
• ponerse = to put on oneself

So me pongo el jersey literally means I put the jumper on myself, and in natural English it often becomes I put on the jumper or I wear the jumper, depending on context.

The reflexive pronoun changes with the subject:

• me pongo = I put on
• te pones = you put on
• se pone = he/she puts on

With clothes, ponerse is extremely common in Spanish.

It can suggest either, depending on context.

In a sentence like this one, No me pongo el jersey verde a no ser que refresque por la tarde, the most natural English translation is often I don’t wear the green jumper unless it gets cooler in the afternoon.

Why? Because the sentence expresses a general habit or condition, not one single action happening right now.

So:

• Me pongo el jersey can mean I put on the jumper
• In a habitual sentence, it can also mean I wear the jumper

Spanish often uses ponerse where English might prefer wear.

El means the, so el jersey verde refers to a specific green jumper, probably one already known in the situation.

• el jersey verde = the green jumper
• un jersey verde = a green jumper

Spanish often uses the definite article with clothing when the speaker has a particular item in mind. So this sentence sounds like the speaker is talking about one specific green jumper they own.

If you said No me pongo un jersey verde..., it would sound more like I don’t wear a green jumper..., meaning any green jumper, not a particular one.

In Spain, jersey usually means jumper or sweater.

This is a useful regional vocabulary point:

• In Spain: jersey = sweater / jumper
• In some other Spanish-speaking places, suéter is more common
• In English, jersey often means a sports shirt, but that is not the meaning here

So for a learner of Spanish from Spain, jersey is a very normal everyday word for a warm knitted top.

In Spanish, adjectives usually come after the noun.

So:

• el jersey verde = the green jumper
• literally: the jumper green

This is the normal word order in Spanish for descriptive adjectives like colours:

• la casa blanca = the white house
• el coche rojo = the red car
• la camisa azul = the blue shirt

So jersey verde is exactly what you would expect.

A no ser que means unless.

It introduces an exception:

• No me pongo el jersey verde a no ser que refresque por la tarde
• I don’t wear the green jumper unless it gets cooler in the afternoon

It is similar to:

• unless
• except if
• only if ... not

A very close alternative is a menos que, which also means unless.

For example:

• No salgo a no ser que me llames.
• I’m not going out unless you call me.

Because a no ser que is followed by the subjunctive, and refresque is the present subjunctive form of refrescar.

This is one of the most important grammar points in the sentence.

• refresca = indicative
• refresque = subjunctive

After expressions like a no ser que and a menos que, Spanish normally uses the subjunctive because the action is not presented as a fact. It is only a possibility or condition.

So:

• a no ser que refresque = unless it gets cooler

The speaker is not saying that it definitely will get cooler — only that this is the condition.

Because after a no ser que, Spanish does not normally use the future tense to talk about a future possibility. It uses the present subjunctive.

English often says:

• unless it gets cooler later
• unless it’s cooler this afternoon

Spanish uses:

• a no ser que refresque

Even though the meaning refers to the future, the grammar is present subjunctive, not future indicative.

This is very common in Spanish after conjunctions that trigger the subjunctive:

• cuando venga = when he comes
• hasta que termine = until it finishes
• a no ser que llueva = unless it rains

This is a spelling change to keep the same sound.

The verb is refrescar. In the subjunctive, forms built from the yo form often change spelling if needed:

• yo refresco
• subjunctive: refresque, refresques, refresque...

Why the change?

In Spanish:

• ce sounds like the / se depending on accent
• que keeps the hard k sound

So refresque is spelled with qu so it still sounds like refreske, not refrese.

The same happens with other verbs:

• buscar → busque
• tocar → toque
• sacar → saque

Here refrescar means to get cooler or to cool down, especially about the weather.

So si refresca or a no ser que refresque means:

• if it gets cooler
• unless it cools down
• unless the weather turns cooler

It does not mean making something cold in a fridge. It is about a change in temperature, usually outdoors.

In this sentence, the idea is that the speaker only wears the green jumper if the afternoon becomes cooler.

Por la tarde means in the afternoon.

So:

• por la mañana = in the morning
• por la tarde = in the afternoon
• por la noche = at night / in the evening

Here it means that the cooling would happen sometime during the afternoon.

A common mistake for English speakers is to think por always means for, but in time expressions like this, por la tarde simply means in the afternoon.

Because Spanish builds this idea as a negative main clause plus an exception.

• No me pongo el jersey verde... = I don’t wear the green jumper...
• ...a no ser que refresque por la tarde = ...unless it gets cooler in the afternoon

This structure is completely normal in both Spanish and English:

• I don’t do X unless Y
• No hago X a no ser que Y

So the no is necessary because the main statement is negative: I do not wear it, except in that one case.

Most naturally, it sounds like a general habit or rule.

• No me pongo el jersey verde a no ser que refresque por la tarde
  = I don’t wear the green jumper unless it gets cooler in the afternoon

This sounds like something the speaker generally does.

Spanish often uses the present tense for habits, routines, and general truths, just like English:

• No tomo café por la noche. = I don’t drink coffee at night.
• Me pongo abrigo si hace frío. = I wear / put on a coat if it’s cold.

So this sentence is not necessarily about today only; it is more likely about the speaker’s usual behaviour.