Usage patterns
確かな〈物〉reliable/trustworthy ~ (e.g. information, method)
I obtained reliable information.
確かに〈事〉to affirm that something is indeed true
確か〈事〉だったと思うif I remember correctly, ~ was the case
If I remember correctly, he was off yesterday.
〈人〉の腕は確かだsomeone's skill is reliable/proven
This craftsman's skill is dependable.
Expressions
気は確かin one's right mind; sane; of sound mind 確かなものにするmake certain; ensure; secure; strengthen