offer加ing: offering
现在分词加ing规则如下：
1)一般情况下直接加ing
think---thinking
sleep---sleeping
study---studying
speak---speaking
carry---carrying
say---saying
2)以不发音的字母e结尾的单词，去掉字母e，再加ing
wake---waking
make---making
come---coming
take---taking
leave---leaving
have---having
3)以重读闭音节结尾，先双写末尾的辅音字母，再加ing。
cut--cutting
get--getting
hit--hitting
4)以y结尾的动词直接加ing
carry——carrying
enjoy——enjoying
5)以ie结尾的动词，把ie改为y，再加ing
die---dying
lie---lying
