And it's actually such an easy one, and it perfectly illustrates the need to ask the following questions, in the following order:
1. What does the word mean?
2. How is it built? What are its relatives?
3. What letters can represent its pronunciation?
If you look into the meaning and structure of <definitely>, you will find words like:
Define (you can get to this word by taking off the <-ly> and the <-ite> suffixes, and we know that this word is a related word by meaning).
In this word we can 'hear' that the letter between the <f> and the <n> is an <i>.
So what if we just take off the <de-> prefix and the <-ly> suffix. This leaves us with:
Finite (Ah-ha - now we can 'hear' the second <i> as well!)
Here's a key point: The spelling will stay the same regardless of how the pronunciation changes!
So, what is the base?
Originally, it would appear to be <fin>, but I know that if it were, that final <n> would double! (Why? Well, because monosyllabic bases that have one consonant at the end and one vowel before that will double before adding a vowel suffix - e.g. swimming, hopping, running etc.).
So, the base must be <fine>! The single silent <e> on the end is dropped when adding a vowel suffix (such as the <-ite> in <finite> and <definite>.
The final <e> in <definite> stays when adding the suffix <-ly>, because <-ly> is a boring old consonant suffix - it doesn't cause <e>s to drop or final letters to double.
Here's a matrix I made up using the Mini-Matrix Maker.
You need to read it from left to right. Not every prefix/base/suffix combination will make a real word. You don't have to use all the word parts, but you can't 'leapfrog' over a column.
From the matrix below, the following words can be made:
The key related word in terms of avoiding the common misspellings of <definitely> is probably <finite>, because it makes it clear that both the first and second syllables have an <i> in them.
And that's definitely a lot easier than just trying to memorise the spelling.
Finally!