SILENT 'E' RULE

This is sometimes called magic or lazy e.
Drop the final e before a vowel suffix: like liking.
BUT keep it before a consonant suffix: like likely

ing is a vowel suffix because it begins with a vowel and ly is a consonant suffix because it starts with a consonant.

The reason is that the vowel in the new ending will keep the first vowel long just as the e did. Drop the e when you do not need it.  N.B. manageable where the e keeps the g soft.
Exceptions: vicious, artificial, racial etc.  The i here does not lengthen the first vowel because it forms a sh sound which alerts you to the exception.

LY as a SUFFIX

A suffix is a small group of letters (here just two) which you tack on to the end of a word to change its usage or meaning.
Ly changes an adjective (which describes a noun) into an adverb (which describes a verb.)
You simply add it on without altering the original word - do not take any letters off and do not attach any new ones. Therefore sincere becomes sincerely, truthful becomes truthfully and so on.
The main exception is true which becomes truly.

With many of these rules there are other minor exceptions but, if you learn the main ones, you will have improved your performance in an examination or in a letter.