Llevo una camiseta fina debajo del jersey cuando hace frío.

Yes—llevar often means to carry / to take, but it also very commonly means to wear / to have on.

So in this sentence, llevo una camiseta fina means I wear / I have on a thin T-shirt.

This is one of the most useful extra meanings of llevar to remember:

• Llevo gafas. = I wear glasses.
• Lleva un abrigo negro. = He/She is wearing a black coat.

Spanish often uses llevar where English uses wear.

Because Spanish normally uses the simple present for this kind of meaning.

Llevo una camiseta fina debajo del jersey cuando hace frío means something like:

• I wear a thin T-shirt under my sweater when it’s cold
• I tend to wear... when it’s cold

Using estoy llevando would sound odd here, because the progressive form is usually for an action in progress right now, and wearing is usually expressed with the simple present in Spanish.

So:

• Llevo un jersey. = I’m wearing a sweater.
• not usually Estoy llevando un jersey for normal clothing description

Here fina means thin, light, or not thick—not fine in the English sense of good or okay.

So una camiseta fina is:

• a thin T-shirt
• a light T-shirt
• possibly an undershirt-like layer, depending on context

It describes the thickness of the fabric.

A learner should be careful here because fino/fina can mean different things depending on context:

• tejido fino = thin/fine fabric
• arena fina = fine sand
• una persona fina = refined/elegant person

In this sentence, it clearly means thin/lightweight.

Because de + el contracts to del.

So:

• debajo de + el jersey → debajo del jersey

This contraction is required in normal Spanish.

Compare:

• debajo del jersey = under the sweater
• debajo de la chaqueta = under the jacket

Only de + el becomes del.
But de + la stays separate:

• debajo de la camisa

Yes, you can.

Both bajo and debajo de can mean under / underneath.

In this sentence:

• bajo el jersey
• debajo del jersey

Both are natural.

A small nuance:

• debajo de often feels a bit more explicitly physical: underneath
• bajo is shorter and very common too

So Llevo una camiseta fina bajo el jersey cuando hace frío is also fine.

In Spain, jersey means sweater / jumper / pullover.

This is a very important vocabulary point because it does not mean the same thing as English jersey in many contexts.

So in Spanish (Spain):

• jersey = sweater / jumper

In other Spanish-speaking regions, people may use other words, such as:

• suéter
• pulóver
• other regional terms

If you are learning Spanish from Spain, jersey for sweater is completely normal.

Cuando hace frío means when it’s cold or whenever it’s cold.

It suggests a regular situation:

• When it’s cold, I wear...
• Whenever it’s cold, I wear...

If you said si hace frío, that would mean if it’s cold, which sounds more conditional:

• If it’s cold, I wear...

Both can be grammatical, but they are not exactly the same:

• cuando = talking about a situation that happens and what you do then
• si = presenting a condition

In this sentence, cuando is the most natural choice for a habitual action.

Because hacer is commonly used in Spanish for weather and temperature expressions.

So:

• Hace frío. = It’s cold.
• Hace calor. = It’s hot.

You generally do not say:

• Es frío for it’s cold (weather)
• Está frío for general weather

Those forms mean different things in other contexts:

• El clima es frío. = The climate is cold.
• La sopa está fría. = The soup is cold.

So for weather, the standard pattern is:

• hace + frío/calor/buen tiempo/mal tiempo

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

• I wear a thin T-shirt under my sweater when it’s cold.

That is because:

• llevo is in the present tense
• cuando hace frío suggests a repeated situation

But depending on context, the Spanish present can sometimes also describe a current situation. Still, without extra context, most learners should understand this sentence as habitual rather than right this second.

Possibly, yes.

Camiseta usually means T-shirt, but in context, if it is worn under a jersey for warmth, English might naturally translate it as:

• thin T-shirt
• undershirt
• base layer

If the speaker wanted to be more explicit, they might say:

• camiseta interior = undershirt

But camiseta fina is perfectly natural if the idea is simply a thin T-shirt layer.

Yes, you could move it, but the original order is the most neutral and natural.

Original:

• Llevo una camiseta fina debajo del jersey cuando hace frío.

This flows naturally:

1. verb: llevo
2. object: una camiseta fina
3. location/position: debajo del jersey
4. time/situation: cuando hace frío

You could also say:

• Cuando hace frío, llevo una camiseta fina debajo del jersey.

That version is also very natural, and it puts more emphasis on the condition when it’s cold.

A less neutral version would be:

• Llevo debajo del jersey una camiseta fina...

That is possible, but it sounds more marked or stylistic.

Yes. Una camiseta means a T-shirt.

It is the indefinite article, like English a/an.

So:

• una camiseta fina = a thin T-shirt

If you removed una, the sentence would not be correct standard Spanish here. Spanish usually needs an article with countable singular nouns in this kind of sentence.

Compare:

• Llevo una camiseta fina. = I’m wearing a thin T-shirt.
• not Llevo camiseta fina in normal standard usage for this meaning

In hace frío, it is best to learn the whole expression as a fixed weather phrase.

Historically and grammatically, frío behaves like a noun-like element in this expression:

• hace frío = literally something like it makes cold, but that is not how you should translate it

For learners, the most practical approach is:

• memorize hace frío = it’s cold
• memorize hace calor = it’s hot

That will help more than trying to force it into an English grammar pattern.