Después de caminar por el bosque, mis botas se llenan de barro.

After después de, Spanish normally uses an infinitive when the subject is the same as in the main clause, or when the subject is not being stated explicitly.

So:

Después de caminar por el bosque...
= After walking through the forest...

If Spanish wants to introduce a full clause with its own subject and tense, it usually uses después de que:

Después de que caminamos por el bosque, ...
= After we walked through the forest, ...

In your sentence, después de caminar is the natural compact structure.

Por is commonly used for movement through, around, or along a place.

So caminar por el bosque means to walk through the forest or to walk around in the forest.

For comparison:

• caminar por el bosque = walk through/around the forest
• caminar en el bosque = walk in the forest
• caminar a través del bosque = walk across/through the forest, often with stronger emphasis on crossing from one side to another

Here, por is very natural because it focuses on movement within that space.

In Spanish, nouns usually need an article more often than in English.

So Spanish prefers:

por el bosque

rather than dropping the article.

English often says through the forest, and Spanish mirrors that with el. Leaving the article out here would sound incomplete or unnatural.

Llenarse de means to get filled with, to become full of, or in many contexts to get covered with.

So:

mis botas se llenan de barro

literally means my boots fill up with mud, but in normal English it often means something like my boots get muddy or my boots get caked with mud.

This is a very common Spanish pattern:

• llenarse de agua = fill with water
• llenarse de polvo = get covered in dust
• llenarse de gente = fill up with people

It is part of the pronominal verb llenarse, but it does not mean the boots are literally doing something to themselves in the way English reflexives work.

A useful way to think of it is:

• llenar algo = to fill something
  El barro llena las botas = The mud fills the boots.
• llenarse de algo = to become filled with something
  Las botas se llenan de barro = The boots get filled with mud.

So yes, it is grammatically a pronominal/reflexive-looking form, but the best way to understand it is as the normal structure of the verb in this meaning.

The subject is mis botas.

That is why the verb is plural:

• mi bota se llena = my boot gets filled
• mis botas se llenan = my boots get filled

The se does not change the agreement. The verb still agrees with the subject, which is botas.

The present tense can describe a habitual or typical result.

So this sentence can mean something like:

After walking through the forest, my boots get muddy.

That sounds like a general truth or something that usually happens.

If you wanted one specific completed past event, Spanish would more likely use the preterite:

Después de caminar por el bosque, mis botas se llenaron de barro.
= After walking through the forest, my boots got muddy.

So se llenan suggests a regular or characteristic outcome, while se llenaron points to one finished occasion.

With llenarse, Spanish normally uses de:

llenarse de barro
llenarse de agua
llenarse de arena

This is just the standard pattern: llenarse de + noun.

Using con here would sound non-standard or unnatural in most contexts. English uses with, but Spanish does not always match English prepositions directly.

Not necessarily.

Spanish llenarse de barro can be more flexible than a very literal English fill up with mud. Depending on context, it can mean:

• mud gets all over them
• they get caked with mud
• they get packed with mud
• they get very muddy

So even if the boots are not literally full inside, the phrase still works naturally.

Yes, both ideas are possible, but they work a little differently.

In your sentence, mis botas is straightforward and clear:

Después de caminar por el bosque, mis botas se llenan de barro.

Spanish often uses the definite article instead of a possessive with body parts and clothing, especially when the owner is shown indirectly:

Después de caminar por el bosque, se me llenan las botas de barro.

That literally looks like the boots fill themselves to me with mud, but the real meaning is:

After walking through the forest, my boots get muddy.

So:

• mis botas = direct, explicit possession
• se me llenan las botas = very natural Spanish way to show the boots belong to me

Both are correct; the second is often especially idiomatic.

Yes. Caminar is perfectly standard and clear, but Spanish has other options depending on nuance.

For example:

• andar por el bosque = walk around / go through the forest
• pasear por el bosque = stroll through the forest

In Spain, andar is very common in everyday speech, but caminar is completely normal and slightly more neutral or standard-sounding in many learning contexts.

So caminar por el bosque is a very good choice here.