Cuando hace frío, me pongo el pijama y me tumbo en el sofá.

In Spanish, weather expressions often use hacer.

So hace frío literally means it makes cold, but in natural English we translate it as it’s cold.

Some very common weather expressions are:

• hace calor = it’s hot
• hace frío = it’s cold
• hace sol = it’s sunny
• hace viento = it’s windy

So for weather, hacer is the normal verb here, not ser.

Here cuando can be understood as when or whenever, depending on context.

In a sentence like this, it often has a habitual meaning:

• Cuando hace frío, me pongo el pijama...
  = When/Whenever it’s cold, I put on my pyjamas...

Because the rest of the sentence is in the present tense, it sounds like a general habit or routine, not just one single occasion.

Ponerse is a reflexive verb, and it often means to put on clothing.

• pongo = I put
• me pongo = I put on myself / I put on

The me shows that the action comes back to the subject. In English we usually just say I put on, but Spanish often uses the reflexive form for clothes:

• Me pongo la chaqueta = I put on my jacket
• Me pongo el pijama = I put on my pyjamas

So me is not optional here if you want this meaning.

Spanish very often uses the definite article (el, la, los, las) with body parts, clothing, and personal items when ownership is already obvious.

So instead of saying my pyjamas, Spanish naturally says:

• me pongo el pijama

Because the me already tells us whose pyjamas they are: the speaker’s.

This is much more natural than me pongo mi pijama, although mi is possible if you really want to emphasize ownership.

Spanish commonly uses el pijama as a singular noun, where English often uses the plural pyjamas.

So:

• el pijama = pyjamas / pajamas

This is just a difference between the two languages. Spanish treats it as one item of clothing.

They are all in the present simple.

• hace = he/she/it does, makes
• pongo = I put
• tumbo = I lie down / I lay myself down

In this sentence, the present tense describes a regular habit:

• Cuando hace frío, me pongo el pijama y me tumbo en el sofá.

This is similar to English: When it’s cold, I put on my pyjamas and lie on the sofa.

Because Spanish usually leaves out subject pronouns when they are clear from the verb form.

• me pongo already tells us the subject is I
• me tumbo also clearly means I

So yo is unnecessary unless you want emphasis or contrast:

• Yo me pongo el pijama, pero mi hermano no.
  = I put on my pyjamas, but my brother doesn’t.

In normal sentences, leaving out yo sounds more natural.

Tumbarse means to lie down, to recline, or to stretch out.

So:

• me tumbo en el sofá = I lie down on the sofa / I stretch out on the sofa

It suggests physically placing yourself in a lying or reclining position.

In this sentence, it gives the idea of getting comfortable on the sofa.

Like ponerse, tumbarse is commonly used as a reflexive verb because the subject is performing the action on themselves.

• tumbar can mean to knock down, to lay something down
• tumbarse means to lie down

Examples:

• Tumbo la silla. = I knock the chair over.
• Me tumbo en la cama. = I lie down on the bed.

So the reflexive form changes the meaning from doing something to another object to placing yourself in that position.

They are related but not the same:

• tumbarse = to lie down / recline
• acostarse = to go to bed / lie down, often with the idea of sleeping
• sentarse = to sit down

In this sentence, me tumbo en el sofá is the best choice because it suggests relaxing on the sofa, probably not going to sleep.

If you said me acuesto en el sofá, it could sound more like I lie down on the sofa with a stronger sense of settling down, possibly to sleep.

Because en is the normal preposition with furniture like sofá, cama, silla, etc. when talking about being on or in that place.

• en el sofá = on the sofa
• en la cama = in/on the bed
• en la silla = on the chair

Spanish uses en much more broadly than English uses in.

Sobre usually means on top of in a more physical or literal sense, and here it would sound less natural.

Because each reflexive verb needs its own reflexive pronoun.

The sentence has two separate verbs:

• me pongo
• me tumbo

So you must say:

• Cuando hace frío, me pongo el pijama y me tumbo en el sofá.

You cannot normally remove the second me, because it belongs specifically to tumbo.

Yes, it is grammatically possible, but the meaning changes slightly.

• me tumbo en el sofá = I lie down / stretch out on the sofa
• me acuesto en el sofá = I lie down on the sofa, often with more of a go to bed / settle down feeling

If the idea is simply relaxing comfortably, me tumbo is a very natural choice.

The accent marks show which syllable is stressed.

• frío → stress on frí
• sofá → stress on fá

They also help with pronunciation and sometimes prevent confusion.

For frío, the accent shows that í-o are pronounced as two syllables: frí-o.

Yes, it sounds natural in Spain.

A Spain learner should especially notice:

• el pijama is very normal
• tumbarse en el sofá is a natural way to say lie down on the sofa
• the whole sentence sounds like an everyday routine

In many Spanish-speaking places outside Spain, it would also be understood perfectly, though people might choose slightly different everyday wording depending on the region.