alcohol => ol is at the end of the word, so if the alcohol group is the most important one, the name ends in ol
If it's a substituent, use 'hydroxy' as there is a 'hydrogen' and 'oxygen' making up 'hydroxy' and 'hydroxy' is like 'hydroxide' except shortened at the end, so it makes sense to put it at the front
oic acid you just have to remember
When naming esters, you have alkyl carboxylate. Example: methyl ethanoate can be thought of as a methyl group bonded to an ethanoate group, which is ethanoic acid minus a hydrogen, for instance
Ketones => 'one' at the back, like propanone; note how 'one' is also at the back of 'ketone'
Aldehydes have 'al', which is a bit weird given that the name begins with 'al'
The two above begin with 'oxo' if not the primary functional group, making sense as these functional groups only consist of oxygen atoms.
Halogen groups are easy xP
Methyl, ethyl etc you just have to know. It's only really up to butyl that you must rote learn as pentyl, hexyl are easier to work out.
Amides are fairly simple as they're named "ethanamide", for instance, with the only real difference being you name substituents attached to the nitrogen as N-methyl, for instance, instead of 2-methyl.
Nomenclature rules? Hmm. Longest hydrocarbon chain is the backbone. Your most important functional group is, well, the most distinguishing feature of your molecule. Substituents are like extra bits and the numbers tell you where they are. Then, order the substituents by alphabetical order. Anyone else you're unclear of?