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

но я точно не знаю

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