Prefiero una mochila cómoda para el día a día.

In Spanish, the subject pronoun is often omitted because the verb ending already shows who the subject is.

• Prefiero = I prefer
• The -o ending tells you it is yo

So Prefiero una mochila cómoda... is completely natural.
You could say Yo prefiero... if you want extra emphasis, for example to contrast your opinion with someone else’s.

Prefiero comes from the verb preferir, meaning to prefer.

This form is:

• first person singular
• present tense

So:

• prefiero = I prefer

It is used when talking about general preferences or what you prefer now.

Because preferir is a stem-changing verb in the present tense.

The e in the stem changes to ie in most forms:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

So prefiero is an irregular present-tense form you simply need to learn.

Una is the feminine singular indefinite article, equivalent to a or one in English.

• mochila is a feminine singular noun
• so it takes una

Examples:

• una mochila
• una maleta
• una chaqueta

If the noun were masculine, you would use un instead.

Because adjectives in Spanish usually agree with the noun they describe in gender and number.

Here:

• mochila is feminine singular
• so the adjective must also be feminine singular
• cómodo → masculine
• cómoda → feminine

So:

• una mochila cómoda = a comfortable backpack

If the noun were masculine, you would say cómodo.

Here, cómoda means comfortable, but in Spanish it can also suggest that something is:

• easy to use
• practical
• convenient
• pleasant to carry or wear

With a backpack, una mochila cómoda usually means a backpack that feels good to carry and is practical for regular use.

Here para means for, in the sense of intended for or suitable for.

So:

• para el día a día = for everyday use / for day-to-day life

It shows the purpose or context in which the backpack is preferred.

El día a día is a very common expression meaning:

• day-to-day life
• everyday life
• the daily routine
• everyday use, depending on context

In this sentence, it refers to normal daily life, not a special trip or event.

So the idea is: the speaker prefers a comfortable backpack for regular everyday use.

Because el día a día works as a fixed noun phrase in Spanish.

Literally, it looks like the day to day, but idiomatically it means everyday life or the daily routine.

Spanish often uses an article where English does not.
So even though the most natural English translation may not include the, Spanish normally does:

• para el día a día

This is the standard expression.

The accent marks show stress and sometimes help mark a word pattern clearly.

• cómoda: the stress falls on the first syllable: CÓ-mo-da
• día: the stress falls on dí, and the accent also shows that í-a is pronounced in two syllables: DÍ-a

Without the accent in día, learners might be tempted to pronounce it as one syllable, but it is two.

Yes. Mochila is the standard and very common word for backpack in Spain.

It can refer to:

• a school backpack
• a travel backpack
• an everyday backpack

Depending on context, Spanish might also use more specific expressions, but mochila is the normal general word.

Yes, you could, but the nuance is a little different.

• Prefiero... = I prefer...
• Me gusta más... = I like ... more

Both can work, but prefiero is more direct and is the most exact match for I prefer.

For expressing a clear preference between options, prefiero is usually the best choice.

The given order is the most natural and neutral:

• Prefiero una mochila cómoda para el día a día.

Spanish word order is somewhat flexible, but changes often sound more marked or emphatic. For example:

• Para el día a día, prefiero una mochila cómoda.

This is also correct, but it puts more focus on for everyday use.

So the original sentence is a very natural standard way to say it.

Not in natural English.
Even though that is close to the literal structure, the natural English translations are:

• for everyday use
• for day-to-day life
• for daily life

This is a good example of an expression that should be understood as a whole, not translated word by word.

It sounds completely natural in Spain.

It is the kind of sentence someone might say when talking about:

• choosing a bag
• shopping
• style versus practicality
• what works best for work, school, or commuting

The phrase para el día a día is especially common in Spain and sounds very idiomatic.