While all learners of English struggle with all the facets of usage and mechanics, Spanish speaking learners deal with specific troubles, most of which arise with concepts that are simple in Spanish but complex and convoluted in English. While one way to express an idea might exist in Spanish, there might be five or six in English, and the particulars of these differences can be a source of confusion and frustration.
Even for native English speakers, homonyms can cause confusion. "There," "their" and "they're"; "effect" and "affect"; "accept" and "except" -- words like these present significant problems for Spanish speakers because in Spanish, for two words to sound identical they would have to be spelled identically, basically being the same word. So it's confusing for Spanish speakers to learn words that sound the same but look different or simply have different meanings that don't seem to follow logical grammatical structure.
While Spanish language uses both definite and indefinite articles, English learners still often struggle with articles. Unlike in English, Spanish speakers don't use articles when addressing unknown amounts of something or abstractions. For example, in the question, "Is there any sugar in the cabinet," the article "any" would be omitted. Also, the difference in usage between "a" and "an," which corresponds to whether the following noun begins with a consonant or a vowel, does not exist with the corresponding Spanish articles "un," "uno," "una," "unos" and "unas."
Negation presents an issue because the rules for usage in Spanish and English are different. In Spanish, you simply use the word "no" in front of a verb to express negation, while in English you have "do not" and "don't," "does not" and doesn't," did not" and "didn't," "have not" and "haven't," and "will not" and "won't," all of which have specific situations for use.
Prepositions are a great source of confusion for Spanish-speaking English learners, because English sometimes uses the same word to convey very different ideas. For example, in English we use the prepositions "in, "on" and "at" to denote time, but in different contexts. Those same words, which adds to the confusion, are also used to denote closed spaces, a surface something is resting upon or a time of day, respectively.