In the case of beside and besides, that single letter `S is the difference between a preposition and an adverb, respectively.
Beside is a preposition that refers to the position `next to` or that signals something as irrelevant. Eg. The test itself is beside the issue; it is the philosophy on which the test is based that must be addressed. Eg. You`ll find `Iron Man` beside the rest of my comic book movies. Eg. That is beside the point.
|To determine if beside is the word you need, substitute the phrase next to or not relevant to for beside in your sentence`if it makes sense, beside is your word.||To determine if besides is the word you need, substitute the phrase in addition to or except for beside in your sentence`if it makes sense, besides is your word.|
|Eg. That is [not relevant to] the point. Besides is an adverb that identifies something as `in addition to` or as an exception.||Eg. Do you have any comments [in addition to] the ones you have already voiced`|
|Eg. Do you have any comments besides the ones you have already voiced`||Eg. No one [except for] the professor can withdraw you from the class.|
|Eg. Besides being a politeness strategy, hedging can also be a means of joining an ongoing conversation.||Eg. I`m too tired to run; [in addition], my running shoes have recently disappeared.|
|Eg. I`m too tired to run; besides, my running shoes have recently disappeared.|
|Eg. No one besides the professor can withdraw you from the class.|