Quando o celular cai no chão, eu fico nervoso.

In this sentence, quando can be understood as when in a general, repeated sense, which in natural English is often closer to whenever.

So:

• Quando o celular cai no chão, eu fico nervoso.
• literally: When the cellphone falls on the floor, I get nervous.
• natural sense: Whenever my cellphone falls on the floor, I get nervous.

Because both verbs are in the present tense, the sentence sounds like a habitual/general truth, not one specific event.

Portuguese often uses the present tense to talk about things that happen generally, habitually, or whenever something occurs.

Here:

• cai = falls
• fico = I get / I become

So the sentence describes a repeated situation:

• Whenever the cellphone falls, I get nervous.

This is normal in Portuguese. English sometimes does the same:

• When it rains, I stay home.
• When my phone falls, I get nervous.

So the present tense here does not mean it is happening right now. It means this is what usually happens in that situation.

Cai comes from the verb cair, which means to fall.

Conjugation:

• eu caio = I fall
• você/ele/ela o celular cai = you/he/she/the cellphone falls

So in the sentence:

• o celular cai = the cellphone falls

A useful thing to notice is that cair is a little irregular in the present tense:

• eu caio
• você/ele/ela cai
• nós caímos
• eles/elas caem

Portuguese uses definite articles much more often than English.

So o celular literally means the cellphone, but depending on context, English may translate it as:

• the cellphone
• my cellphone
• a cellphone

In Portuguese, using the article here sounds very natural. Saying just celular in this sentence would sound incomplete or unnatural.

This is very common:

• O carro quebrou. = The/My car broke down.
• A chave sumiu. = The/My key disappeared.

So o celular is normal everyday Portuguese.

No is a contraction of:

• em + o = no

So:

• no chão = on the floor / onto the floor depending on context

In this sentence:

• cai no chão = falls on the floor / falls to the floor

This kind of contraction is extremely common in Portuguese:

• em + a = na
• em + os = nos
• em + as = nas

Examples:

• na casa = in the house
• no carro = in the car
• nos livros = in the books

So em o chão is not correct in normal Portuguese; it should be no chão.

Good question. Prepositions do not match perfectly from one language to another.

In Portuguese, em can cover meanings that English expresses with in, on, or at, depending on the context.

So:

• no chão literally uses em, but the natural English translation is on the floor

Other examples:

• na mesa = on the table
• na parede = on the wall
• no trabalho = at work

So it is best not to translate em word-for-word every time. Learn the full expression:

• cair no chão = to fall on/to the floor

This is about the meaning of ficar.

Ficar often means to become or to get when talking about a change of state.

So:

• eu fico nervoso = I get nervous / I become nervous

Compare:

• sou nervoso = I am a nervous person
  This describes someone's general personality.
• estou nervoso = I am nervous
  This describes the current state.
• fico nervoso = I get nervous
  This emphasizes the change that happens in that situation.

In your sentence, the idea is:

• the cellphone falls
• then I become nervous

That is why ficar fits very well.

No, eu is not strictly necessary.

You could say:

• Quando o celular cai no chão, fico nervoso.

That is still correct, because the verb fico already shows the subject is I.

Portuguese often drops subject pronouns when the verb form makes the subject clear.

Why include eu then?

• for emphasis
• for clarity
• for contrast

For example:

• Quando o celular cai no chão, eu fico nervoso, mas minha irmã não.

Here eu helps emphasize I.

So both are possible:

• Quando o celular cai no chão, eu fico nervoso.
• Quando o celular cai no chão, fico nervoso.

Because nervoso is an adjective, and it describes the speaker’s state.

• fico nervoso = I get nervous

You use an adjective after ficar to describe what someone becomes.

Compare:

• fico feliz = I get happy
• fico triste = I get sad
• fico cansado = I get tired

Nervosamente means nervously, which is an adverb, so it would describe how someone does something, not what state they are in.

Example:

• Ele fala nervosamente. = He speaks nervously.

That is different from:

• Ele fica nervoso. = He gets nervous.

Yes. The adjective should agree with the speaker’s gender.

• A man would usually say: eu fico nervoso
• A woman would usually say: eu fico nervosa

So if a female speaker says the sentence, it would be:

• Quando o celular cai no chão, eu fico nervosa.

This agreement happens with many adjectives in Portuguese:

• cansado / cansada
• preocupado / preocupada
• animado / animada

Yes, it could be reversed.

The sentence starts with the when-clause:

• Quando o celular cai no chão, then
• eu fico nervoso

This is very natural and common.

You could also say:

• Eu fico nervoso quando o celular cai no chão.

This means the same thing. The difference is mostly about emphasis and style.

• Starting with Quando... puts the situation first.
• Starting with Eu fico nervoso... puts your reaction first.

Both are correct.

Yes, celular is the normal everyday word in Brazil for cell phone / mobile phone.

In European Portuguese, people more commonly say:

• telemóvel

So:

• Brazil: celular
• Portugal: telemóvel

If you are learning Brazilian Portuguese, celular is exactly the word you want.

By itself, it most naturally sounds like a general habit or repeated situation:

• Whenever the cellphone falls, I get nervous.

If you wanted to talk about one specific event in the past, Portuguese would normally use past forms, for example:

• Quando o celular caiu no chão, eu fiquei nervoso.
• When the cellphone fell on the floor, I got nervous.

So the original sentence is best understood as a general statement, not one single past event.

Not always. Chão can mean:

• floor indoors
• ground outdoors
• sometimes soil/earth depending on context

In your sentence, no chão is naturally on the floor if you are indoors, or on the ground if the context is outside.

So the exact English word depends on the situation, but the Portuguese phrase stays the same:

• cair no chão

Yes, absolutely.

• Quando o celular cai no chão... = When the cellphone falls...
• Quando meu celular cai no chão... = When my cellphone falls...

Using meu makes possession explicit. Without meu, Portuguese often relies on context, and the article o is enough.

Both are natural, but they can feel slightly different:

• o celular = more general or context-based
• meu celular = specifically my phone

Usually, yes. Cair generally means to fall, which normally suggests that it happened accidentally, not intentionally.

So:

• o celular cai no chão usually means the cellphone drops/falls to the floor

If someone intentionally throws it, Portuguese would more naturally use another verb, such as:

• jogar = to throw
• derrubar = to knock/drop something

For example:

• Eu derrubei o celular no chão. = I dropped the cellphone on the floor.
• Ele jogou o celular no chão. = He threw the cellphone on the floor.

So cair usually presents the phone as the thing that falls, not the person who causes it.

The comma separates the introductory quando clause from the main clause:

• Quando o celular cai no chão,
• eu fico nervoso.

This is very common and helps readability.

Portuguese punctuation here works much like English:

• When the cellphone falls on the floor, I get nervous.

If you reverse the order, the comma is often not used:

• Eu fico nervoso quando o celular cai no chão.

So the comma is there because the time clause comes first.