To convey an attitude to an action or any event you should use special words: должен. рад, уверен, готов. надо/нужно.
The words рад, уверен, готов are short forms of adjectives that's why they agree with nouns or pronouns in gender and numeral.
Look at the structure of the following sentences:
For example:
BUT
Я уверен, что видел этот фильм.
Remember!