Anoche un mosquito me dejó una picadura en el brazo, y tuve que evitar rascarme.

Anoche means last night. It is a time expression, and Spanish often places time expressions at the beginning of the sentence to set the scene.

So:

• Anoche = last night
• Un mosquito me dejó una picadura... = a mosquito left me a bite...

You could also say:

• Un mosquito me dejó una picadura en el brazo anoche

But starting with Anoche sounds very natural.

Un mosquito means a mosquito, so it introduces the mosquito as something not specifically identified beforehand.

Use un when it is:

• not previously mentioned
• not a specific known one
• just one example of that thing

Use el mosquito if both speaker and listener already know which mosquito is meant, or if you are talking about it as a specific one already mentioned.

Here, un mosquito is the natural choice because it means some mosquito / a mosquito bit me.

Yes, it is a natural way to express the result of the bite.

Literally, me dejó una picadura means:

• it left me a bite / a bite mark

So the focus is not just on the action of biting, but on the consequence: there was a bite left on the arm.

A more direct way to say a mosquito bit me would be:

• Un mosquito me picó

That is probably the most straightforward version.

But me dejó una picadura is also natural because it emphasizes the mark or irritation left behind.

They are related, but they are different parts of speech:

• picó = bit / stung → a verb
• picadura = bite / sting / bite mark → a noun

So:

• Un mosquito me picó = A mosquito bit me.
• Un mosquito me dejó una picadura = A mosquito left me with a bite.

The second one focuses on the bite itself as a thing.

That me means to me / on me.

Spanish often uses an indirect object pronoun when something happens to a person or affects them.

So:

• me dejó una picadura = it left me a bite
• more naturally in English: it left a bite on me

That me shows who was affected.

Compare:

• Me rompió el vaso = He broke my glass / He broke the glass on me.
• Me manchó la camisa = He stained my shirt.

Spanish often uses this structure where English might prefer a possessive like my or simply rephrase it.

Both are possible, but en el brazo is very common and natural in Spanish.

Spanish often uses:

• an indirect object pronoun (me, te, le, etc.)
• plus the definite article (el, la, los, las)

instead of a possessive adjective like my.

So:

• me dejó una picadura en el brazo = literally, it left me a bite on the arm
• natural English: it left me a bite on my arm

Because me already tells us whose arm it is, Spanish usually does not need mi.

This is very common with body parts:

• Me duele la cabeza = My head hurts.
• Me lavé las manos = I washed my hands.
• Me golpeé el brazo = I hit my arm.

Dejó is the preterite of dejar. The preterite is used here because the mosquito bite is viewed as a completed event in the past.

So:

• dejó = left
• one completed action last night

Likewise, tuve is also preterite because I had to is also presented as a completed past event in that situation.

This sentence is narrating specific events that happened last night, so the preterite is the normal tense.

Both can mean I had to, but they are not exactly the same.

• tuve que = I had to, as a completed event or specific obligation
• tenía que = I had to / I was supposed to / I needed to, often describing an ongoing situation, background, or repeated obligation

In this sentence, tuve que evitar rascarme means there was a specific moment or situation in which I had to stop myself from scratching.

So tuve que fits well because it refers to a particular need that arose last night.

Compare:

• Tuve que ir al médico = I had to go to the doctor.
  (specific completed event)

• Cuando era niño, tenía que ir al médico a menudo = When I was a child, I had to go to the doctor often.
  (habitual or ongoing)

Evitar rascarme means to avoid scratching myself. It suggests making an effort not to do it.

That is slightly different from simply saying not to scratch myself.

• Tuve que no rascarme is not natural Spanish.
• Tuve que evitar rascarme is the natural structure.

You can think of it as:

• I had to avoid scratching myself
• I had to stop myself from scratching

So evitar + infinitive is a very useful pattern.

Examples:

• Evita tocarte la cara = Avoid touching your face.
• Tuve que evitar reírme = I had to avoid laughing.

Because rascarse means to scratch oneself.

Here:

• rascar = to scratch something / someone
• rascarse = to scratch oneself

So:

• Tuve que evitar rascarme = I had to avoid scratching myself

The me is attached to the infinitive because Spanish allows object/reflexive pronouns to attach to infinitives.

You could also say:

• Tuve que evitarme rascar

But that sounds unnatural here. The normal form is:

• evitar rascarme

In Spanish, with an infinitive, pronouns are often attached directly to the infinitive.

So:

• rascarme = scratch myself

This is very common:

• quiero verlo = I want to see it
• necesito levantarme = I need to get up
• debo cuidarme = I should take care of myself

Putting the pronoun before evitar would change the structure and would not work naturally in this sentence.

Not normally for a mosquito.

In Spanish, insects like mosquitoes usually pican, not muerden.

So:

• Un mosquito me picó = correct and natural
• Un mosquito me mordió = generally not the usual choice

Very roughly:

• picar is used for bites/stings from insects and similar creatures
• morder is used more for animals that bite with their mouths, like dogs or people

That is why the noun is picadura here.

It can be changed somewhat, because Spanish word order is flexible.

The original sentence is natural:

• Anoche un mosquito me dejó una picadura en el brazo, y tuve que evitar rascarme.

But you could also hear variations like:

• Anoche, un mosquito me dejó en el brazo una picadura...
• Un mosquito me dejó una picadura en el brazo anoche...

Some versions sound more natural than others depending on emphasis, but the original order is smooth and standard.

Spanish often moves elements around for focus, but learners should usually stick with the most neutral order unless they have a reason to change it.

It would be understood everywhere in the Spanish-speaking world.

Nothing in it is uniquely Spain-only in a way that would cause confusion.

That said, different regions might prefer slightly different ways of phrasing it. For example, many speakers might more simply say:

• Anoche un mosquito me picó en el brazo y tuve que evitar rascarme.

That version is also very natural and perhaps more direct.

But the original sentence is perfectly understandable and grammatical across the Spanish-speaking world.