El autobús va lleno por la mañana, así que prefiero ir a pie.

Because autobús is a masculine noun in Spanish, so it takes el.

A lot of Spanish nouns ending in -o are masculine and many ending in -a are feminine, but autobús doesn’t follow that pattern, so you just have to learn its gender with the word.

• el autobús
• los autobuses

A good habit is to learn nouns together with their article.

In this sentence, va lleno is a very natural way to say that the bus goes along full / is packed as it runs, especially as a usual situation.

With transport, Spanish often uses ir + adjective to describe the condition something is in while moving or functioning:

• El autobús va lleno
• El tren va vacío
• La carretera va muy lenta (less literal, but similar idea in some contexts)

If you said está lleno, that would also be understandable, but it sounds more like you are simply describing its state at a particular moment. Va lleno feels more idiomatic here for a bus during its route.

Lleno agrees with autobús, which is masculine singular, so it stays lleno.

Compare:

• El autobús va lleno
• La guagua va llena
• Los autobuses van llenos
• Las guaguas van llenas

So the ending changes depending on the noun it describes.

In Spain, por la mañana is the normal way to say in the morning.

Spanish commonly uses:

• por la mañana
• por la tarde
• por la noche

Using en la mañana is much less common in Spain and may sound regional or less natural in this context.

Also, don’t confuse it with de la mañana, which is often used with clock times:

• a las siete de la mañana

So:

• por la mañana = during the morning
• de la mañana = of the morning, often after a time

Así que introduces a result or conclusion. It works like so, therefore, or which is why in English.

Here it connects the two ideas:

• the bus is packed in the morning
• as a result, the speaker prefers to go on foot

It is very common in everyday Spanish.

Similar connectors include:

• por eso = that’s why / because of that
• entonces = then / so
• de modo que = so / therefore

In this sentence, así que sounds very natural and conversational.

Because Spanish often drops subject pronouns when the verb form already shows who the subject is.

Prefiero clearly means I prefer, so yo is not necessary.

You could say:

• Yo prefiero ir a pie

but that usually adds emphasis, contrast, or clarity. For example:

• Yo prefiero ir a pie, pero mi hermano no.

Without special emphasis, leaving out yo sounds more natural.

Both are possible, but they are not exactly the same.

• ir a pie = to go on foot
• caminar = to walk

Ir a pie focuses on the method of getting somewhere. It contrasts naturally with going by bus, car, train, etc.

So in this sentence, prefiero ir a pie is especially appropriate because the speaker is choosing a mode of transport.

Prefiero caminar would also be grammatical, but it can sound slightly more focused on the physical action of walking rather than the transport choice.

Because ir a pie is a fixed Spanish expression meaning to go on foot.

Spanish uses different prepositions in transport expressions, and they are often just idiomatic:

• ir a pie
• ir en autobús
• ir en coche
• ir en tren

So a pie is simply the standard expression you need to learn as a whole phrase.

Yes, in many situations you can say el bus, and people will understand you. It is common in many Spanish-speaking places.

However, in Spain, autobús is a very standard and neutral choice. Depending on the region, you may also hear other words such as:

• bus
• autocar (usually more like coach, not city bus)
• guagua (not standard in most of mainland Spain, but used in places like the Canary Islands)

For a learner of Spanish from Spain, autobús is a safe default.

The accent mark shows where the stress falls: au-to-BÚS.

Without the written accent, Spanish pronunciation rules would make you stress it differently. The accent mark tells you that the final syllable is stressed.

It also helps when forming the plural:

• autobús
• autobuses

The stress changes naturally in the plural form, and the written accent is no longer needed there.

The comma separates the two linked clauses:

• El autobús va lleno por la mañana
• así que prefiero ir a pie

In Spanish, it is very common to put a comma before connectors like así que when they introduce a consequence or conclusion.

It helps show the pause and makes the sentence easier to read.

Not in exactly the same way.

• porque means because and introduces a reason
• así que means so and introduces a result

This sentence is structured as:

1. cause: the bus is packed in the morning
2. result: the speaker prefers to go on foot

So así que is the right connector here.

If you wanted to use porque, you would need to change the structure, for example:

• Prefiero ir a pie porque el autobús va lleno por la mañana.

Both versions are natural, but the relationship is expressed from a different angle.

In practice, it tells you when the bus is full, so it mainly goes with va lleno:

• El autobús va lleno por la mañana

That then explains the speaker’s preference in the second part.

So the timeline is:

• in the morning, the bus is full
• therefore, the speaker prefers to go on foot

Grammatically, it sits in the first clause, but logically it motivates the whole statement.