Вчём разница между at the weekend и at weekends. и как правилльно будет at weekends или at weekend?

At the weekend - в выходные дни недели  at weekends - по выходным.  at weekends  -

There are twenty-one desks in the classroom and two pupils sit at each desk. the teacher’s desk is in front of the pupils’ desks. the blackboard is on the wall on the teacher’s left. the chalk and the duster are in a box near the blackboard. there are four very large windows on the side of the room opposite the door. we open the windows during the break between lessons. there are portraits on the wall over the blackboard. there are pictures and maps hanging on the walls in the classroom.   в классной комнате двадцать одна парта, и за каждой партой сидят по два ученика. учительский стол находится перед партами учеников. доска находится на стене, слева от учителя. мел и тряпка в коробке возле доски. на стороне напротив двери четыре больших окна. мы открываем окна во время перемены между уроками. на стене над доской висят портреты. в классной комнате на стенах висят картины и карты.

1. write the following verbs in the past simple. go-went -gone, play-played- played , see -saw -seen , watch-watched- watched , have-had -had , swim -swam- swum , be-was /were -been , wash-washed- washed , read-read -read , ride-rode -ridden , run -ran -run , skate-skated : skated , buy-bought- bought , come-came- come , do-did -done , rain -rained -rained , snow-snowed -snowed , wear-wore -worn , put-put -put , get-got -got , give-gave- given , take-took -taken , say-said- said . 2. complete the sentences. use present simple or past simple. 1) bill usually gets up at 7 o'clock but yesterday he got up at 8 o'clock. 2) every morning we swim in the swimming pool but last week we swam in the river. 3) it often rains in spring. 4) two months ago they traveled to italy. 5) every evening sally has tea with a cake but yesterday she had milk with honey.