Both of the girls are ...
結構︰Both/Three/Some/Many/All of the+複數名詞(或代名詞)+複數動詞+…。
說明︰此句型意為“…其中兩個(三個，一些，許多，全部)…”。both，three，some，many，all都是<不定代名詞>;若跟隨 of 表示限定範圍的為複數<代名詞>，則<代名詞>前不加 the。該結構中<動詞>須用複數。
Both of the girls are nice. 這兩位女孩是好的。
Many of the supermarkets are having sales. 很多這些超級市場正在舉行拍賣。
Three of the students came here yesterday. 這些學生中有三位昨天來這裡。

... who ...
結構︰主詞(人)+關係代名詞(Who)+動詞+…。
說明︰who 是主格<關係代名詞>，引導<形容詞子句>，前面的先行詞接“人”，後面接<動詞>，必須和先行詞保持一致。who 可用 that 代替。
The man who called was tired. 打電話的那個男人是疲倦的。
The boy who is speaking to Helen enjoys playing soccer.
正在跟海倫談話的那男孩喜愛踢足球。
People who use their free time well are usually healthy and happy.
善於利用空閒時間的人通常健康又快樂。

... which ...
結構︰主詞+動詞+名詞(物)+關係代名詞(which)+動詞…。
說明︰which 可作主格或受格<關係代名詞>，引導<形容詞子句>，前面的先行詞接“動物”或“事物”，後面接<動詞>，必須和先行詞保持一致。
Here is a book, which is very interesting. 這裡有一本非常有趣的書。
I like to watch TV programs which are about sports.我喜歡觀賞有關運動的電視節目。
They want to sell the house, which has only one door.他們想賣掉那幢只有一扇門的房子。
... by oneself ...
結構︰主詞(人，物)+動詞+by oneself…。
說明︰此句型意為“某人(物)自己…”。反身<代名詞>的人稱、數和性別，須和相關的<主詞>一致。
They went to Suao by themselves. 他們自己去蘇澳。
John fixed the tape recorder by himself. 約翰靠他自己修理錄音機。
You can’t go mountain climbing by yourself. 你不能自己去登山。