Basic talks about something that is essential or simple.
Basis talks about something that is the starting point or most important part of something.
- I have a basic understanding of English tenses.
(simple understanding) - Reading, writing and spelling are the basic communication skills needed for life.
(reading, writing and spelling are essential skills) - This information formed the basis of their opinion.
(important part to create their opinion) - They’ve been working on the basis that they wouldn’t need more money.
(from the start they believed they wouldn’t need more money)
