• You (speak, speaks) English very well.
• We (study, studies) Russian.
• I (sit, sits) only on this chair.
• He (clean, cleans) the blackboard every day

1- speak
2-studies
3-sit
4-cleance

You speak English very well.
We study Russian.
I sit only on this chair.
He cleans the blackboard every day.
Если подлежащее стоит в 3-ем лице единственного числа ( he , she, it , то к сказуемому добавляется -s, или -es:
He cleans
She speaks
It goes ( вместо местоимений могут быть имена )
Аnn speaks