Attention or attention means notice or consideration, or mental concentration --
Raise your hand to get her attention.
Please pay attention; this is important.
Attention is also a military command, meaning to stand up straight and look forward --
Company, attention!
Remain at attention until you hear him say "at ease".
Inattention or inattention means neglect, or failure to notice or consider --
His inattention in matters of dress makes prospective employers wary.
He shows up for class, but his inattention is obvious.
Now that you know that, you can say --
"He is failing because of his inattention to lectures; when he pays attention he's a good student."
Note --"attentions" is an old-fashioned term meaning courting behavior --
He's been paying his attentions to the young widow next door.
