When my twentith birthday came,I was not at home.In order to celebrate such an important day with my parents,I wrote them a letter.
1.“第二十”拼写错误,应该是twentith
2.so的用法错误,属于混淆了such与so
正确的表达方式为：so+adi+a/an+n
such+a/an+adj+n
3.“父母”拼写错误.应为parents.
parent的意思为 父亲（或母亲）,先辈,起源,根源,保护者
只有它的复数名词parents才表示父母的意思.
还有几个比较重要的语法点就是
1.in order to do,in order that+从句
2.write sb a letter=write （a letter）to sb=drop sb a line 给某人写信