Learn Portuguese (Portugal): Perco o autocarro, mas não posso ficar aqui.

The word mas translates to "but". It is a coordinating conjunction used to connect two contrasting ideas. In this case, it contrasts the problem (Perco o autocarro) with the speaker's urgent need to leave (não posso ficar aqui).

Does the verb perder mean "to lose" or "to miss"?

It means both! You can use perder for losing an object (like your keys) or for missing transport, like o autocarro (the bus). Also, notice that its "I" form is slightly irregular: instead of "perdo", it becomes perco.

Why is the second verb, ficar, left in its full dictionary form?

In Portuguese, when you use a helper verb like poder (can / to be able to) followed by another action, only the first verb is conjugated. We conjugate the first verb to match the speaker (posso means "I can"), and leave the main action verb in its infinitive form (ficar, meaning "to stay").

Why is the word for "I" (eu) missing before the verbs?

In Portuguese, the subject pronoun is often dropped because the verb itself tells you who is acting. The forms perco (I miss) and posso (I can) already show that the subject is "I", so you do not need to include eu in the sentence.