Prefiero ir andando a la biblioteca cuando hace buen tiempo.

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

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

So prefiero is completely natural. You can say yo prefiero if you want extra emphasis or contrast, for example:

• Yo prefiero ir andando, pero mi hermano prefiere ir en coche.

Prefiero comes from the verb preferir, meaning to prefer.

It is a stem-changing verb in the present tense:

• preferir
• yo prefiero
• tú prefieres
• él/ella prefiere

The e in the stem changes to ie in many forms. This is very common in Spanish.

So:

• preferir → prefiero

Ir andando is a very common way to say to go walking, or more naturally in English, to go on foot / to walk there.

In this sentence:

• Prefiero ir andando a la biblioteca
  = I prefer to walk to the library

It focuses on the way you go somewhere: by walking.

In Spain, ir andando is very natural everyday language.

Because the sentence is about going to a destination, not just the general action of walking.

• andar / caminar = to walk
• ir andando = to go somewhere by walking

So:

• Voy andando al trabajo = I go to work on foot
• Camino mucho = I walk a lot

You could also say:

• Prefiero caminar a la biblioteca

That is understandable and correct, but ir andando sounds very natural when the idea is getting somewhere by walking.

Andando is the gerund of andar.

The structure ir + gerund can sometimes mean to keep doing something, but here ir andando is a set, natural expression meaning to go on foot.

So this is not really the same as English I am going walking. It is better understood as:

• go by walking
• go on foot

Both can mean to go on foot.

• ir andando = very common, conversational, natural in Spain
• ir a pie = also correct, sometimes a bit more neutral or formal

Examples:

• Prefiero ir andando a la biblioteca.
• Prefiero ir a pie a la biblioteca.

Both work. In Spain, ir andando is especially common in everyday speech.

The preposition a is used with verbs of movement to show the destination.

• ir a = to go to

Since biblioteca is feminine singular, a + la biblioteca becomes:

• a la biblioteca

So:

• ir a la biblioteca = to go to the library

Spanish often uses hacer for weather expressions.

• hace frío = it is cold
• hace calor = it is hot
• hace buen tiempo = the weather is nice / it is good weather

So hace buen tiempo is a fixed, very common expression. It does not literally match English grammar, but it is the normal Spanish way to say it.

Bueno becomes buen before a singular masculine noun.

• un buen libro
• buen tiempo

Since tiempo is masculine singular, Spanish uses buen here.

So:

• hace buen tiempo = correct
• hace bueno tiempo = incorrect

It can mean either, depending on context, but in this sentence it most naturally means whenever the weather is nice.

The whole sentence expresses a general habit or preference:

• Prefiero ir andando a la biblioteca cuando hace buen tiempo.

So the idea is:

• I prefer to walk to the library when the weather is nice
• or more naturally, whenever the weather is nice

The present tense in Spanish is often used for habits, routines, and general preferences.

So prefiero here does not mean only right now, at this moment. It means something like:

• In general, I prefer...
• Usually, I prefer...

That is why the present tense is the natural choice.

Yes. Spanish word order is flexible, though some versions sound more natural in certain contexts.

For example, you could also say:

• Cuando hace buen tiempo, prefiero ir andando a la biblioteca.

This puts more emphasis on the condition when the weather is nice.

The original version is perfectly natural too:

• Prefiero ir andando a la biblioteca cuando hace buen tiempo.

Yes, but the meaning changes slightly.

• Prefiero ir andando... = I prefer to walk...
• Me gusta más ir andando... = I like walking there more...

Preferir sounds a bit more direct and clear for comparing options. It suggests a choice:

• walking rather than taking the bus, driving, etc.

So prefiero is the best choice if you want to express preference clearly.