Some of the students are not sure ___ spelling the word, while others are not sure ___ the use of some grammatical rules.
The heavy snow stopped the farmers ______ in the fields.
He was named Robert _____ his father by mother.
Tom kept quiet about the accident ____ lose his job.
He wrote to his friends ______ information.
Old McDonald gave up smoking for a while, but soon _____ to his old way.