I am confused by the vocab lesson for 借りる (かりる) . At the top it lists it as かりる , meaning to borrow or to have a loan. It gives no alternates or anything like that. However, none of the examples except the last use this. The rest all just use かり instead (the last example uses both forms). There is no reason given for why the る is dropped, or when it would be included. In the review, it shows かります and かりるます as alternates in the sentence: " I will borrow a pencil from a friend." but the lesson gives no information on which to use when. These are both polite forms, and the same tense. Am I missing something here, and there is a conjugation rule around る-verbs that I am forgetting? I do know that when it occurs at the end of a sentence the る is typically dropped and replaced with the ending (like ます, ません, ない, etc) however, the examples are dropping the る in the middle of sentences, not at the end
I just looked through all the example sentences and they are just using different grammar forms. like past or te-form, etc. If these are troubling you, I would suggest learning a bit more grammar first. Maybe look through the N5 grammar, which forms are troubling for you. All of those examples are very common verb conjugations.
The beginning is always hard, and it’s difficult to tell what to learn first: vocab or grammar. I wish you the best 
Ok thanks! I am using the default course load Bunpro suggested (3 new grammar and 10 new vocab per day). Right after posting this, I just got the る verb (past) course that explains る is dropped and replaced with た (or ほした for polite) past tense. In case anyone else comes across this issue, this is the course that explains it る-Verb (Past) (JLPT N5) | Bunpro. I didn’t think it was a tense thing due to the “Review” showing both forms as alternates in the given sentence and not marking it as different (usually it’ll mark it as negative, polite, etc when the alternate has a reason other than formality).
As I said, that’s sadly just very natural to struggle a bit in the beginning. There is just sooooo much to learn at once! So feel free to ask here anytime you got a question.
It’s great you are working with the sentences and try to understand them. A lot of new learner skip this step, cause it’s hard. But if you at least try to read it (even if you won’t grasp all the sentences) you will skill up way faster
I’ll also add that on vocab reviews like this, bunpro will usually accept the base form of verbs even if the sentence requires a different conjugation (because it is not a grammar review). This is why @maedhros77, in the first example sentence, “かりるます” is considered an “alternate” answer even though the real correct one is “かります”. If I’m not mistaken (I’m still not very advanced), “かりるます” is incorrect japanese, the verb can be conjugated as “かれる” (base, casual form) or “かります” (polite form). But when you review the word, bunpro asks you to complete a sentence presented like this: “[…]を〇ます。”[1], the ます is already there, so the only characters missing are “かり”, to complete the polite form “かります”. But since it’s a vocab review, the system also accepts your answer if you wrote the full base form of the verb, “かりる”, without getting the right conjugation, and it ends up being displayed as “かりるます” (because the ます part was already there), but the sentence is actually wrong if written like that.
This detail is not important to know if you got the answer right or wrong though, it’s only worth having in mind when and if you re-read the whole sentence afterwards to make sure you don’t internalise incorrect japanese (and after you’ve actually studied some of the conjugation involved, so feel free to ignore this whole message if that doesn’t make any sense 
and maybe come back later to it if you’re curious).
[1]: the 〇 indicates where you need to write something.
I remember that when starting Bunpro from scratch, the vocab lessons would accept the dictionary form of the verb (かりる) every time. I had no knowledge of verb conjugation, so that worked well. At one point, it stopped taking those as correct answers (?). I think it’s a nice transition, but it’s unclear to me if it’s on purpose (it must be) and at what point the transition happens. There is no message or something similar to communicate this.
Another feature I noticed is that as you start a new grammar deck (e.g. N5 > N4), the lower level vocab reviews start showing phrases of the new grammar level. Again, a nice transition but without clear communication.
Ok, good to know. Thanks for the heads up