Si no encuentro un taxi, vuelvo a casa andando.

This is a very common Spanish pattern.

Spanish often uses:

si + present tense, present tense

to talk about a real future condition:

• Si no encuentro un taxi, vuelvo a casa andando.
• Si tengo tiempo, te llamo.

In natural English, we often use if + present, will + verb:

• If I don’t find a taxi, I’ll go back home on foot.

Spanish can also use the future in the second part:

• Si no encuentro un taxi, volveré a casa andando.

That is also correct, but the present tense in the main clause sounds very natural and common.

Because Spanish usually drops subject pronouns when they are not needed.

Both verbs already show that the subject is I:

• encuentro = I find
• vuelvo = I return / go back

So yo is unnecessary unless you want emphasis or contrast:

• Si no encuentro un taxi, vuelvo a casa andando. = neutral
• Si no encuentro un taxi, yo vuelvo a casa andando. = more emphatic, like I’m the one who goes back walking

Because encontrar is a stem-changing verb.

In the present tense, its o changes to ue in most forms:

• encuentro
• encuentras
• encuentra
• encontramos
• encontráis
• encuentran

So encuentro is the correct yo form.

For the same reason: volver is also a stem-changing verb.

Its o changes to ue in most present-tense forms:

• vuelvo
• vuelves
• vuelve
• volvemos
• volvéis
• vuelven

So vuelvo is the correct form.

Yes.

Both are correct:

• Si no encuentro un taxi, vuelvo a casa andando.
• Si no encuentro un taxi, volveré a casa andando.

The version with vuelvo is very natural in everyday Spanish and can sound a bit more immediate or matter-of-fact.

The version with volveré is slightly more explicitly future.

Because Spanish often uses casa without an article when it means home.

So:

• volver a casa = to go back home
• llegar a casa = to arrive home
• estar en casa = to be at home

But la casa is used when you mean a specific house as a building:

• Vuelvo a la casa de mi amigo. = I’m going back to my friend’s house.

So in your sentence, a casa means home, not just to the house.

Andando is the gerund of andar.

Here it means walking or on foot. It tells you how the person goes back home.

So:

• vuelvo a casa andando = I go back home walking / on foot

In Spain, this use is very natural. It is similar to saying:

• vuelvo a casa a pie

Both are correct. Andando often feels especially everyday and conversational.

Both can mean on foot, but they are structured differently.

• andando is a gerund: it focuses on the action of walking
• a pie is a fixed expression: it means on foot

So these are both fine:

• Vuelvo a casa andando.
• Vuelvo a casa a pie.

In this sentence, andando sounds very natural in Spain.

Because in Spanish, singular countable nouns usually need an article or determiner.

So encontrar un taxi means to find a taxi.

Saying just encuentro taxi would sound incomplete or unnatural in standard Spanish.

You could change the determiner depending on the meaning:

• un taxi = a taxi
• el taxi = the taxi
• ningún taxi = no taxi

But you normally need something there.

Yes, this is a very common confusion.

Here it is si no = if not

• Si no encuentro un taxi... = If I don’t find a taxi...

Written as one word, sino usually means but rather / except:

• No voy en taxi, sino andando. = I’m not going by taxi, but rather on foot.

So in your sentence, it must be si no because it introduces a condition: if not.

Because si without an accent means if.

Spanish distinguishes:

• si = if
• sí = yes

So in this sentence:

• Si no encuentro un taxi... = If I don’t find a taxi...

No accent is correct.

It is normal and helpful here because the si clause comes first:

• Si no encuentro un taxi, vuelvo a casa andando.

When the conditional clause comes before the main clause, a comma is commonly used.

If the order is reversed, Spanish often leaves the comma out:

• Vuelvo a casa andando si no encuentro un taxi.

So the comma here is standard and natural.

Yes.

You can also say:

• Vuelvo a casa andando si no encuentro un taxi.

That means the same thing.

The version with the si clause first often sounds slightly more natural when you want to present the condition first. The second version puts more focus on the result.

Both are correct.