Se o novo não funcionar, eu vou usar o antigo.

Se means if. It introduces a condition:

• Se o novo não funcionar = If the new one doesn’t work
• eu vou usar o antigo = I’m going to use the old one / former one

So the whole sentence is a normal if + result structure.

In Portuguese, when a conditional clause comes first, it is very common to separate it with a comma:

• Se o novo não funcionar, eu vou usar o antigo.

This is similar to English:

• If the new one doesn’t work, I’ll use the old one.

If the second part came first, the comma would often disappear:

• Eu vou usar o antigo se o novo não funcionar.

Here, novo and antigo are adjectives being used as nouns. This is very common in Portuguese.

• o novo = the new one
• o antigo = the old one / the former one

Portuguese often uses article + adjective where English says the ... one.

Compare:

• o novo celular = the new phone
• o novo = the new one

The noun is understood from context and gets omitted.

It actually is conjugated — but this is a tricky point.

After se referring to a future possibility, Portuguese usually uses the future subjunctive:

• Se ele for... = If he goes / if he is...
• Se eu tiver... = If I have...

With the verb funcionar, the future subjunctive form is funcionar, which looks exactly like the infinitive.

So in this sentence:

• Se o novo não funcionar
  the form is future subjunctive, not a plain infinitive.

A comparison helps:

• Se o novo não funcionar...
• Se ele não for bom...
• Se eu não tiver tempo...

Only some verbs show a visible difference.

In Portuguese, não normally goes directly before the verb it negates.

So:

• não funcionar = not work
• não vou usar = I’m not going to use

In your sentence, the negative belongs to the first clause:

• Se o novo não funcionar = If the new one doesn’t work

Both are possible, but eu vou usar is much more common in everyday Brazilian Portuguese.

• eu vou usar = I’m going to use
• usarei = I will use

In Brazil, the structure ir + infinitive is very natural and frequent in speech. The simple future (usarei) is correct, but it can sound more formal, written, or less conversational.

So:

• Se o novo não funcionar, eu vou usar o antigo. → very natural in speech
• Se o novo não funcionar, usarei o antigo. → correct, but more formal

Not strictly. Portuguese often allows the subject pronoun to be omitted when the verb already makes the subject clear.

So both are correct:

• Se o novo não funcionar, eu vou usar o antigo.
• Se o novo não funcionar, vou usar o antigo.

Including eu can add clarity, emphasis, or just reflect the speaker’s style. In Brazilian Portuguese, subject pronouns are used more often than in European Portuguese, so eu sounds completely natural.

Not exactly.

• antigo often means former, previous, or old in the sense of the earlier one
• velho usually means old in the sense of aged / worn out

In this sentence, o antigo most likely means the old one as in the previous one, not necessarily something physically old.

Compare:

• o antigo celular = the old/previous phone
• o celular velho = the old/worn-out phone

A native speaker might choose antigo here because it contrasts nicely with novo and suggests the previous one.

Yes, but it changes the nuance.

• o antigo = the old/previous one
• o velho = the old one, often implying it is aged, old-fashioned, or worn

So:

• Se o novo não funcionar, eu vou usar o antigo.
  usually sounds like: If the new one doesn’t work, I’ll use the previous one.

• Se o novo não funcionar, eu vou usar o velho.
  can sound more like: If the new one doesn’t work, I’ll use the old worn one.

Both are grammatical, but antigo is often the better choice for a simple contrast with novo.

Because Portuguese often leaves out a noun when it is already obvious from context.

For example, if everyone knows you are talking about a phone, device, system, or method, you can simply say:

• o novo = the new one
• o antigo = the old one

This is similar to English, but Portuguese does it very naturally with articles plus adjectives.

Full version:

• Se o aparelho novo não funcionar, eu vou usar o aparelho antigo.

Shorter natural version:

• Se o novo não funcionar, eu vou usar o antigo.

Yes. The article and adjective would change to match the gender of the omitted noun.

Masculine:

• Se o novo não funcionar, eu vou usar o antigo.

Feminine:

• Se a nova não funcionar, eu vou usar a antiga.

For example, if the missing noun were feminine, like a máquina or a versão, you would use a nova / a antiga.

Yes. A very common alternative is:

• Eu vou usar o antigo se o novo não funcionar.

This means the same thing. The main difference is emphasis and rhythm:

• Se o novo não funcionar, eu vou usar o antigo.
  starts with the condition
• Eu vou usar o antigo se o novo não funcionar.
  starts with the result

Both are natural.

It is neutral and very natural in everyday Brazilian Portuguese.

Nothing in it is especially slangy or especially formal. In fact:

• Se o novo não funcionar... is standard
• eu vou usar... is very common spoken Brazilian Portuguese
• o antigo sounds normal and clear

So this is a good sentence to learn as a practical real-life pattern.