Is this correct to use a comma in this situation (for animals and
their owners, I have learned...)?
Does it make sense to use surprising in this sentence? ("It seemed surprising to me that beside of famous and notorious toxins...")
Shouldn't water start with a capital letter? ("...many essential elements and substances like Calcium, Sodium and even water can be considered toxins.")
The answer to 1) is 'yes'. We can use a comma here because there are two parts to the sentence. The main clause is:
- I have learned that poisoning was one of the most common problem which we had to deal with it.
The phrase at the beginning:
- After [ starting my education in Veterinary Medicine and beginning to know diseases and issues that cause problems for animals and their owners ]
... is an adjunct giving extra information. It contains a full clause or 'mini-sentence'. When adjuncts like this come at the beginning of the sentence, we sometimes use a comma to separate them from the main part of the sentence (but not always). It is more likely if the adjunct is very, very long.
This sentence is special kind of sentence called an 'extraposition'. That pronoun at the beginning, it, doesn't have any meaning. We arrange the sentence like this because it is easier for the reader to get the complicated information at the end of the sentence, not the beginning. The sentence means:
- [that many essential elements and substances like Calcium, Sodium and even water can be considered toxins] is surprising to me.
The word surprising is fine in this sentence.
Some writers sometimes put the names of chemical elements in capital letters: Oxygen, Nitrogen and so forth. However, water is not an element. It is also not a very scientific word - everybody knows what water is. So we definitely don't need a capital letter for water. I agree with the Original Poster, that it looks strange to have Calcium and Sodium in capital letters in the same list as water. Maybe it would be better for the writer to use small letters for all the words instead of capitals. People who write about chemicals in academic or professional publications usually don't recommend using capitals for elements. (See comment by David Richerby below.)
Hope this is helpful!