Learn Portuguese (Portugal): O autocarro é mais lento do que o comboio.

In English, we often add '-er' to the end of short words (like slow becoming slower). In Portuguese, you don't change the ending of the adjective for comparisons. Instead, you always put mais (more) in front of it. So 'slower' is literally translated as 'more slow' (mais lento).

What does do que mean in this sentence?

do que translates to 'than' when you are comparing two things. The core formula you are learning here is mais [adjective] do que (more [adjective] than). Note that in casual spoken Portuguese, people sometimes drop the do and just say que, but do que is the complete, standard way to say it.

Why does the sentence say do que o comboio instead of just do que comboio?

In Portuguese, you generally need to use the definite article (o or a, meaning 'the') before a noun, even in comparisons. Just like the sentence starts with o autocarro (the bus), it must finish with o comboio (the train).

Why do we use é instead of está for 'is' in this comparison?