I learned Spanish in high school and am relearning it now. I have also been studying Mandarin Chinese for the past two years. While I find conjugating to be a bit of a pain to learn, I do appreciate not having to deal with that in Chinese. That said, you do have to deal with getting the tones right. If you don't, your Chinese does not sound like Chinese (which is something my Chinese wife often says about me).
Also, there are many words that are quite similar in Spanish to English. There are very few words in Chinese that are similar to English. English and Spanish have very similar grammars and sentence structure. Both languages inflect questions the same way.
In Mandarin, the grammar is very different. After two years, this is still a constant struggle. That is probably the hardest thing about the language. One basic rule is never translate literally, word for word. Also, if you try to use Google Translate to translate a sentence from English to Chinese, you are likely to get something that sounds strange to Chinese speakers.