首先说一下，不是“sarf”而是“scarf”，意思是“围巾”
原句：Mrs Smith buys a scarf.（史密斯夫人买了一条围巾。）
变为否定句是：Mrs Smith does not buy a scarf.（史密斯夫人没有一条围巾。）
scarf
英 [skɑːf] 美 [skɑːrf]
n. 围巾，披巾，头巾；（嵌接的）斜面，斜嵌槽；鲸脂的切口
v. <美，非正式>狼吞虎咽地吃；嵌接，斜接（木材，金属）；切割（鲸）脂
[ 复数 scarves或scarfs 第三人称单数 scarfs 现在分词 scarfing 过去式 scarfed 过去分词 scarfed ]
双语例句：
Her scarf streamed behind her.
她的围巾在身后飘动。
Someone has taken my scarf.
有人把我的围巾拿走了。
She tied her scarf over her head.
她把围巾围在头上。
肯定句改否定句的方法：
1、be动词后加not，诸如：is not(isn't)， are not(aren't)， was not(wasn't)， were not(weren't)， am not（无缩写）。
◆ He is a student. 他是一个学生。--- He is not a student. 他不是一个学生。
◆ They were watching TV then. 那时他们正在看电视。--- They weren't watching TV then. 那时他们没在看电视。
2、情态动词等后加not，诸如：can not(can't)， will not(won't)，should not(shouldn't)，must not(mustn't)，need(needn't)等。
◆ She can take care of herself. 她能照顾好她自己。--- She can not(can't) take care of herself. 她不能照顾好她自己。
◆ You should go home. 你应该回家。--- You shouldn't go home. 你不应该回家。
3、若无法找到be动词或情态动词，就在已有动词前额外加助动词否定形式：don't/doesn't/didn't，同时将动词变原形。
◆ She likes the room. 她喜欢这房间。--- She doesn't like the room. 她不喜欢这房间。
◆ We did our homework yesterday. 我们昨天做了作业。--- We didn't do our homework yesterday. 我们昨天没有做作业。
注：句中若有some ，须将其改成any。
◆ I want some books. 我想要一些书。--- I don't want any books. 我不想要任何书。
◆ There is some water in the cup. 杯子里有一些水。--- There isn't any water in the cup. 杯子里没有水了。
否定句，Mrs Smith doesn't buy a scarf.
英文翻译如下
史密斯夫人买了一件纱丽本回答被网友采纳