Breakdown of Ты работаешь быстро, а он — медленно.
он
he / it (for a masculine noun) / it (for a neuter noun)
а
and (contrastive)
ты
you (singular)
работать
to work
Questions & Answers about Ты работаешь быстро, а он — медленно.
Why do быстро and медленно end in -о?
Why does the sentence use а for "but" instead of но?
The word а is a contrastive conjunction. It acts like "whereas" or "while" in English to compare two different subjects doing different things (you do this, whereas he does that). The word но is used for direct contradictions or exceptions, like "I want to work, but I am tired."
What is the dash doing in the second half of the sentence?
The dash replaces the repeated verb работает (works). Instead of saying а он работает медленно, Russian often drops the second verb and uses a dash to show the omission. It sounds much more natural this way!
Just to review, why is the verb spelled работаешь?
This is the present tense form of работать (to work) that matches the informal "you" (ты). For this group of verbs, you drop the -ть from the dictionary form and add the ending -ешь.