O bastão de caminhada caiu.

O is the masculine singular definite article, meaning the.

So o bastão = the stick/pole.

In Portuguese, articles are used more often than in English. With a specific object like this, o sounds completely natural.

• o = the (masculine singular)
• a = the (feminine singular)
• os = the (masculine plural)
• as = the (feminine plural)

Because bastão is a masculine noun, it takes o.

You usually have to learn the gender together with the noun: o bastão.

Although many Portuguese nouns ending in -ão are masculine, the ending alone is not a perfect rule for every word. The safest habit is to memorize nouns with their article:

• o bastão
• a caminhada

That way you naturally learn both meaning and gender at the same time.

De caminhada describes what kind of stick it is.

So:

• bastão = stick / pole / cane
• bastão de caminhada = walking stick / hiking pole

The structure noun + de + noun is very common in Portuguese and often works like English combinations such as:

• walking stick
• coffee cup
• train station

More examples:

• sapatos de corrida = running shoes
• garrafa de água = water bottle
• óculos de sol = sunglasses

Here, de caminhada means something like for walking/hiking.

Cair is the infinitive, meaning to fall.

Caiu is the past tense form used here: fell.

More specifically, it is the 3rd person singular of the pretérito perfeito simples:

• eu caí = I fell
• tu caíste = you fell
• ele/ela/você caiu = he/she/you fell

Since the subject is o bastão de caminhada (a singular thing), Portuguese uses caiu.

It is the pretérito perfeito simples, which is commonly used for a completed past event.

So caiu means the fall happened and is seen as a finished action:

• O bastão caiu. = The stick fell.

This tense is often used for single completed events in the past.

Because the subject is already stated: O bastão de caminhada.

Portuguese does not need an extra subject pronoun if the noun is present. In English, we might say:

• The walking stick fell.

We do not also say:

• The walking stick it fell. ✘

It is the same idea here. The noun phrase itself is the subject, so no extra ele is needed.

In careful pronunciation, caiu is roughly ka-YOO, with two vowel sounds.

A simple learner-friendly approximation is:

• caiu ≈ kah-YOO

In European Portuguese, unstressed vowels are often reduced, so in natural speech it may sound more compressed than you expect.

Also:

• ca- from cair
• -iu is a common ending in past forms

If you are learning Portugal Portuguese specifically, listen for how speakers shorten unstressed vowels compared with Brazilian Portuguese.

It is natural and understandable, especially for a hiking or trekking context.

Depending on context, you may also hear other words:

• bastão = pole, stick
• bengala = cane / walking stick, often more like a support cane
• bastão de trekking = trekking pole
• bastão de caminhada = walking stick / hiking pole

So this sentence sounds fine, especially if you mean a walking or hiking pole rather than a medical cane.

Because de caminhada is not an adjective here. It is a prepositional phrase built with de + noun.

So this is not the same type of structure as:

• o carro vermelho = the red car

Instead, it is:

• o bastão de caminhada = the walking stick / hiking pole

Portuguese often uses de + noun where English uses a noun directly in front of another noun.

Yes. That would mean:

• A walking stick fell
• or One walking stick fell, depending on context

The difference is:

• O bastão de caminhada caiu. = The walking stick fell.
  A specific walking stick.
• Um bastão de caminhada caiu. = A walking stick fell.
  Not previously identified, or not important which one.

So the article changes how specific the noun is.

The sentence follows a very common Portuguese order:

Subject + verb

• O bastão de caminhada = subject
• caiu = verb

So:

• O bastão de caminhada caiu.

This is the same basic order as in English:

• The walking stick fell.

It is a simple and neutral sentence structure.