Esta noche me quedo en casa porque estoy cansado.

Because quedarse is the usual verb for to stay in this sense.

• quedar can mean things like to remain, to be left, or to arrange to meet
• quedarse often means to stay / to remain somewhere

So:

• Me quedo en casa = I’m staying at home
• Quedo en casa would not sound right here for that meaning

The me is part of the verb quedarse. It does not literally mean myself in English here; it is just how this verb works.

Conjugation:

• me quedo = I stay / I’m staying
• te quedas = you stay
• se queda = he/she stays

En casa means at home.

Spanish very often uses casa without an article in this expression:

• Estoy en casa = I’m at home
• Me quedo en casa = I’m staying at home

This is normal and idiomatic.

Compare:

• en casa = at home
• en la casa = in the house

En la casa sounds more like you are talking about the physical building, while en casa is the usual expression for the idea of home.

Yes. Esta noche is a very common and natural way to say tonight.

Literally it is this night, but in normal English we translate it as tonight.

Examples:

• Esta noche salgo. = I’m going out tonight.
• Esta noche me quedo en casa. = I’m staying home tonight.

In Spain, esta noche is standard and very common.

Because cansado here describes a temporary state: tired.

Spanish usually uses:

• estar for states or conditions
• ser for identity, inherent characteristics, or classification

So:

• Estoy cansado = I’m tired
• Soy cansado would mean something more like I am tiring / I am a tiring person, which is a very different idea and usually not what you want

This is one of the classic ser vs estar differences.

Only cansado changes to match the speaker’s gender:

• male speaker: Estoy cansado
• female speaker: Estoy cansada

The rest stays the same:

• Esta noche me quedo en casa porque estoy cansada.

Me quedo does not change for gender.

Because porque is the word for because.

Here are the key differences:

• porque = because
• por qué = why

So:

• Me quedo en casa porque estoy cansado. = I’m staying home because I’m tired.
• ¿Por qué te quedas en casa? = Why are you staying home?

This is a very common spelling question for learners.

Spanish often leaves out subject pronouns when they are not needed.

The verb already tells you who the subject is:

• me quedo = I stay / I’m staying

So yo is optional.

• Me quedo en casa = normal, neutral
• Yo me quedo en casa = adds emphasis, contrast, or clarity

For example:

• Ellos salen, pero yo me quedo en casa. = They’re going out, but I’m staying home.

So omitting yo is completely normal.

Because Spanish often uses the present tense for a planned or near-future action, just like English sometimes does.

• Esta noche me quedo en casa = Tonight I’m staying home

This is a very natural way to talk about a plan or decision.

You could also say:

• Esta noche me voy a quedar en casa = Tonight I’m going to stay home

Both are correct. The version with the present tense is very common and sounds natural.

Yes, but it depends on the structure.

Quedarme is the infinitive form with the attached pronoun, so you use it after another verb or expression:

• Voy a quedarme en casa. = I’m going to stay home.
• Prefiero quedarme en casa. = I prefer to stay home.

But when the verb is conjugated, the pronoun normally goes before it:

• Me quedo en casa.

So:

• me quedo = correct finite verb form
• quedarme = correct infinitive form, but not by itself as the main verb in this sentence

Spanish word order is fairly flexible, although the original order is very natural.

The original sentence:

• Esta noche me quedo en casa porque estoy cansado.

Other possible versions:

• Me quedo en casa esta noche porque estoy cansado.
• Porque estoy cansado, esta noche me quedo en casa.

These all work, but they may sound slightly different in emphasis.

The original version is a very natural, neutral way to say it:

• time first: Esta noche
• main action: me quedo en casa
• reason: porque estoy cansado

Yes, but it changes the feel slightly.

• en casa = at home
• en mi casa = in my house / at my home

En casa is more idiomatic and is usually the best choice when you simply mean at home.

Use en mi casa when you want to be more explicit or contrast it with someone else’s home:

• Hoy me quedo en mi casa, no en la tuya. = Today I’m staying at my place, not yours.

So in your sentence, en casa is the most natural option.

Yes. Cansado is an adjective meaning tired.

It agrees with the person it describes:

• Estoy cansado = I’m tired (male speaker)
• Estoy cansada = I’m tired (female speaker)
• Estamos cansados = We’re tired (masculine or mixed group)
• Estamos cansadas = We’re tired (all-female group)

In this sentence, cansado describes the speaker, so it must match the speaker’s gender.