[会发现] 句①
My parents
作主语。句②
I
作主语。句③
To see
作主语。句④
It
做形式主语，真正的主语是：
to meet friends in person from time to time, not just on social media
。句⑤
Smoking
作主语。句⑥
What he has said
作主语。
答案：My parents; I; To see; It; to meet friends in person from time to time, not just on social media; Smoking; What he has said
解析：句①“My parents are worried...”中主语是“My parents”；句②“I'm a new senior...”中主语是“I”；句③“To see is to believe.”中主语是“To see”；句④“It's important to meet...”中“it”是形式主语，真正主语是“to meet friends in person from time to time, not just on social media”；句⑤“Smoking is bad...”中主语是“Smoking”；句⑥“What he has said is true.”中主语是“What he has said”。
[会发现] 句①由单一动词
had
做谓语。句②复合谓语：系动词＋表语结构
was
difficult
和
was
kind
and
friendly
作谓语。句③复合谓语：情态动词＋实义动词结构
had
to
think
作谓语。
答案：had; was difficult; was kind and friendly; had to think
解析：句①“I just had my first maths class...”中谓语是单一动词“had”；句②“The class was difficult, but the teacher was kind and friendly.”中复合谓语是“was difficult”和“was kind and friendly”；句③“First, I had to think very carefully...”中复合谓语是“had to think”。
