Keep (something) straight (in one's mind/head) phrase
To be capable of understanding something completely, clearly, or precisely.
You should present your idea more clearly so that everyone can keep it straight in their mind.
She found it hard to keep yesterday lesson straight in her head.
To be able to differentiate one thing from another.
Can you figure out how to keep these two paintings straight? They look the same to me.
The verb "keep" should be conjugated according to its tense.
Meaning absolutely certain or without a doubt
I am as sure as God made little green apples that Peter's the one for you, you two make a beautiful couple.