I wanted to watch the anime but I was wondering if it adapted the Light novel closely. If not, then I plan to read the Light Novel before watching it.
So, this is a difficult question to answer because for the most part, the anime covered the important points. As far as I know, season 1 of the anime covered it sufficiently and left out trivial things. They did switched out studios for season 2, however. Season 1 was done by Brain Base and season 2 was done by Studio Feel.
With that said, there are a lot of things that the first episodes of S2 did not explain that are highly important. You can see a detail of it in this detailed Reddit post. In summary, it explains to non-LN readers why Yukinon is acting the way she is. LN readers would understand why and they can see the little things like facial expression and other things that might be overt. Non-LN anime watchers would have to be very attentive to capture the information otherwise.
This isn't a simple yes or no answer. If you don't have the time to read the LN then I recommend you watch the anime as it is, as far as I know, comprehensive. If you care about details or were confused by some of things going on, then either read the LN or read the analysis for the episodes.
Yes and no. Some important scenes was modified or even cut, in order to make people interpret diffently, or to make watchers who want to know the full story, decide to buy light novel. I think like this because I noticed that it might be that certain person planned brilliantly in advance to modify scenes in anime to increase influence of watchers to look for original reference.
If you already read light novel, you can try in-depht Oregairu analysis to explore thoughts & feelings of some characters