This is the 1095th most frequent Spanish word.
usar
To use.
In this sentence, 'uso' is the first-person singular present tense of 'usar', meaning 'I use.' It denotes the act of utilizing a pencil as an instrument for writing.
Yo uso un lápiz para escribir.
I use a pencil to write.
Here, 'usan' is the third-person plural present tense form, meaning 'They use,' which in this context implies that they wear comfortable shoes, hence indirectly using them for practicality.
Ellos usan zapatos cómodos.
They wear comfortable shoes.
The word 'usar' in this question remains in its infinitive form and denotes the act of operating the machine, placed after the auxiliary verb 'saber' (to know).