rigid

definition: 
Unable to bend or be forced out of shape; not flexible
sentence: 
The cabin has plenty of storage spaces, but the door pockets would be much more useful with flexible sides instead of rigid ones.
difficulty: 
very easy
toefl frequency: 
2
toefl classification: 
adverb: 
rigidly
adjective: 
rigid
adverb sentence: 
He adhered rigidly to his opinions about marriage.
adjective sentence: