Si no encuentro la tiza, usaré el rotulador azul.

Because after si meaning if, Spanish normally uses the present indicative, not the future.

So:

• Si no encuentro la tiza... = If I don’t find the chalk...
• ...usaré el rotulador azul. = ...I’ll use the blue marker.

This is a very common pattern in Spanish:

• Si tengo tiempo, iré. = If I have time, I’ll go.
• Si llueve, nos quedaremos en casa. = If it rains, we’ll stay at home.

Using si no encontraré would sound wrong here.

Here si no is two separate words:

• si = if
• no = not

So Si no encuentro la tiza means If I don’t find the chalk.

This is different from sino as one word, which usually means but rather / except / rather than.

Compare:

• Si no encuentro la tiza, usaré el rotulador azul. = If I don’t find the chalk, I’ll use the blue marker.
• No usaré tiza, sino rotulador. = I won’t use chalk, but rather a marker.

That distinction is very important.

Because Spanish often leaves out subject pronouns when the verb ending already shows who the subject is.

Here:

• encuentro = I find
• usaré = I will use

So yo is understood automatically.

You could say:

• Si no encuentro la tiza, usaré el rotulador azul.

Or, if you want emphasis:

• Si no encuentro la tiza, yo usaré el rotulador azul.

But the version without yo is the most natural in everyday Spanish.

Because Spanish nouns normally use articles much more often than English does.

• la tiza: tiza is a feminine noun
• el rotulador: rotulador is a masculine noun

Even when English might say chalk or a blue marker without much thought about articles, Spanish often includes them:

• la tiza
• el rotulador azul

In this sentence, the articles help make the objects sound specific and natural.

In Spanish, most descriptive adjectives usually come after the noun.

So:

• el rotulador azul = the blue marker

This is the normal order:

• la casa blanca = the white house
• el coche rojo = the red car

Putting azul before the noun would sound unusual here and would usually only happen for special stylistic or poetic effect.

Usaré is the first person singular simple future of usar.

So:

• usaré = I will use

The simple future is formed by adding endings to the infinitive:

• usar
• usaré
• usarás
• usará
• usaremos
• usaréis
• usarán

In this sentence, it matches the English idea of a future result:

• If I don’t find the chalk, I’ll use the blue marker.

The accent mark in usaré shows where the stress goes: u-sa-RÉ.

It also helps distinguish forms clearly. In the future tense, the -é ending for yo normally has an accent:

• usaré
• comeré
• viviré

So the accent is not optional here; it is part of the correct spelling.

Yes. In many everyday situations, both are possible:

• Si no encuentro la tiza, usaré el rotulador azul.
• Si no encuentro la tiza, voy a usar el rotulador azul.

Both mean roughly the same thing.

A small difference in feel:

• usaré can sound a bit more neutral or straightforward
• voy a usar can sound a bit more conversational or immediate

In ordinary speech, both are natural.

Yes. Rotulador is a very common word in Spain for a felt-tip pen or marker.

In other Spanish-speaking countries, different words may be more common, such as:

• marcador
• plumón

So if you are learning Spanish from Spain, rotulador is absolutely the right word to know.

It is the normal and recommended punctuation here.

The sentence begins with a conditional clause:

• Si no encuentro la tiza

Then comes the main clause:

• usaré el rotulador azul

Spanish usually separates that initial clause from the main clause with a comma:

• Si no encuentro la tiza, usaré el rotulador azul.

You may sometimes see commas omitted in informal writing, but the comma is the standard choice.

In most of Spain, z is pronounced with the th sound from English think.

So azul sounds approximately like:

• a-THOOL

Also, the u is a pure Spanish vowel, not like the English oo changing shape.

If you are learning Spain Spanish, that th pronunciation is the expected one. In many Latin American varieties, it would be pronounced with an s sound instead.

Yes, but it changes the nuance.

• la tiza suggests a specific piece of chalk, or the chalk that is expected in the situation
• una tiza means a piece of chalk, less specific

So:

• Si no encuentro la tiza... = If I don’t find the chalk
• Si no encuentro una tiza... = If I don’t find a piece of chalk / any chalk

In a classroom-type context, la tiza sounds very natural because it often refers to the chalk already understood in the situation.

Yes, a little, but the original sentence is the most natural.

For example, you could say:

• Usaré el rotulador azul si no encuentro la tiza.

This is also correct and means the same thing.

The difference is mainly focus and rhythm:

• Starting with Si no encuentro la tiza puts the condition first
• Starting with Usaré el rotulador azul puts the action first

Both are valid, but the original version is a very standard way to express this idea.