English is a notoriously difficult language to learn according to people who picked it up later in life. There are lots of rules and words that sound similar, but mean very different things. Even us native speakers can get tripped up and make these usage mistakes... which we see every day on Facebook, am I right?
"Invariably" is sometimes used to mean frequently, but it actually means something that happens every time. Meanwhile "Intensive purposes" isn't what you meant to say. The expression is actually "for all intents and purposes."
"Insure" and "Ensure" sound similar but refer to different things. To ensure means you don't want something to happen, while insure means you do.
Same goes for "Accept" and "Except," where the X in the word is your clue. Except means to cut something out, where accept means to receive.