No encuentro mi subrayador amarillo en la mochila.

In Spanish, no goes before the conjugated verb to make the sentence negative.

• Encuentro = I find / I am finding
• No encuentro = I don’t find / I can’t find

So No encuentro mi subrayador amarillo... means I can’t find my yellow highlighter...

Spanish does not usually use a helper verb like do in negatives the way English does.

Encuentro is the first-person singular present tense of encontrar (to find).

• yo encuentro = I find
• in this kind of sentence, it often translates more naturally as I can’t find

Even though it is present tense, Spanish often uses the present where English might use can’t find in everyday speech.

In Spanish, possessive adjectives like mi, tu, su, nuestro normally go before the noun and do not use an article with it.

So:

• mi subrayador = my highlighter
• not el mi subrayador

This is different from English only in structure, not meaning.

Subrayador can mean highlighter, but vocabulary varies a lot across the Spanish-speaking world.

Common possibilities include:

• subrayador
• resaltador
• marcador
• marcatextos

In Latin America, resaltador is very common in many places.
So this sentence is fine, but depending on the country, a native speaker might choose a different word.

In Spanish, most color words and many adjectives usually come after the noun.

So:

• subrayador amarillo = yellow highlighter
• literally: highlighter yellow

This is the normal word order in Spanish.

The adjective must agree with the noun in gender and number.

• subrayador is masculine singular
• so the adjective is amarillo

Compare:

• el subrayador amarillo
• la mochila amarilla

Because mochila is feminine, its adjective would be amarilla, but here amarillo is describing subrayador, not mochila.

It describes subrayador.

So the sentence means:

• I can’t find my yellow highlighter in the backpack

not

• I can’t find my highlighter in the yellow backpack

If Spanish wanted to clearly describe the backpack as yellow, it would usually say:

• No encuentro mi subrayador en la mochila amarilla.

Spanish usually uses the definite article (el, la, los, las) more often than English does.

So en la mochila is the natural way to say:

• in the backpack
• or sometimes in my backpack, if the context already makes it clear whose backpack it is

Even when English might omit an article in some situations, Spanish often keeps it.

Spanish often uses the where English prefers my, especially when the owner is obvious from context.

So en la mochila can sound natural even if the speaker means their own backpack.

Spanish does this a lot with possessions when context makes ownership clear.

For example:

• Me duele la cabeza = My head hurts
• literally: The head hurts me

Likewise, en la mochila can naturally refer to the relevant backpack without repeating my.

Yes. That is also correct.

• en la mochila = in the backpack
• dentro de la mochila = inside the backpack

Dentro de is a bit more explicit.
In everyday speech, en la mochila is simpler and very common.

Because the personal a is only used before specific people (and sometimes pets or personified beings), not ordinary objects.

So:

• No encuentro mi subrayador amarillo = no a
• No encuentro a mi hermano = yes, because my brother is a person

Since a highlighter is a thing, there is no personal a here.

Yes, but the original order is the most natural.

Standard order:

• No encuentro mi subrayador amarillo en la mochila.

You might also hear:

• No encuentro en la mochila mi subrayador amarillo.

That version puts more focus on where the speaker has looked.
But for a learner, the original sentence is the best neutral pattern to follow.

They are very similar, but not always exactly the same.

• No encuentro... = I can’t find... / I’m not finding...
• No puedo encontrar... = I can’t manage to find... / I’m unable to find...

No encuentro is shorter and very natural in everyday speech.
No puedo encontrar can sound a little more emphatic.

In this sentence, either one could work, but No encuentro is very common.

The sentence already sounds natural. A speaker might also say:

• No encuentro mi resaltador amarillo en la mochila.
• No hallo mi subrayador amarillo en la mochila. (less common in casual speech in many places)
• No veo mi subrayador amarillo en la mochila. (more like *I don’t see my yellow highlighter in the backpack)*

The biggest regional change would probably be the word for highlighter, not the grammar.