[文法] 請問一句文法

[livingminus (あっぱれ！)]

想問的是最後一句, 但我想把整段打出來會比較清楚明白
希望有人能為我解答, 謝謝

For many young people sport is a popular part of school life and being in
one of the school teams and playing in matches is very important.  If someone
is in a team it means a lot of extra practice and often spending a Saturday
or Sunday away from home, as many matches are played then.
                          --------------------------------

請問,為什麼要用are played呢?

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※ 發信站: 批踢踢實業坊(ptt.cc) 
◆ From: 125.233.6.245
推 LunacyAlma:match當名詞用，比賽、對戰
→ LunacyAlma:週六日通常是比賽開打的日子，所以會在這時離家去外地
推 stranck:因為 matches 是複數，所以要用 are...
→ stranck:而因為這裡的 matches 是被動 (被 play)，所以用 played
→ stranck:附帶：then 就是指 a Saturday or Sunday...
→ stranck:　　　　　　　　　　　　　　　　　　　↑↑are played
→ livingminus:非常感謝樓上的好人。m(_ _)m