Features of an L2 are of varying difficulty. For example, many English learners of Spanish struggle with things like subjunctive mood. Personally, I struggle with negation in Norwegian (the negative particle comes after the verb).
What influences how hard something is to learn in a second language?
Roughly four factors determine how hard something is to learn (agnostic of L1): frequency, salience, redundancy, and context, each of which I'll discuss below. These features work together to determine how easy something is to learn, none works in isolation.
Of course, features that are similar between your L1 and L2 will be easier to acquire than ones that are very different. I've lost the reference at the moment, but Spanish plural marking (basically identical to English) is easier to acquire for L1 English-speakers than plural marking in, say, French (plurals in French have more forms, and are additionally difficult to hear).
The present progressive ending "-ing" is one of the first things acquired in L2 learners of English, and will be running example throughout.
Frequency is a measure of how often you hear something. Raw frequency alone doesn't determine easiness (after all, "the" is incredibly common, but speakers of some L1s still struggle with it), but Collins et al. (2009) showed that "-ing" occurred more often with more frequent verbs, possibly driving acquisition.
Some things are harder to hear than others. For example, "-ing" is the only major derivational suffix in English that is a whole syllable: /ɪŋ/ (cf. "-s" /s/ and "-ed" /d/). It also has a clear meaning (maybe some English L1 bias sneaking in here), compared to an ending like "-en" ("eaten") or "-s" which is almost meaningless.
Goldschneider and DeKeyser (2001) argue salience is overall the most important factor, including both prominence of meaning and phonetic form.
Redundancy refers to the fact that there are multiple ways to express any one meaning in a language. For example, "I will study tonight" vs. "I am studying tonight" (said in the afternoon). It's hard to acquire an abstract feature that is not always expressed the same way.
Polysemy is the notion of having multiple meanings. For example the English suffix "-s," variously realized as /s/, /z/, or /ɨz/ can mean plural, possessive, or third person singular subject.
Touched on briefly in the Frequency section, some things appear in more contexts, like the "-ing" ending, known as semantic scope. Basically, it easier to break down the meaning of something the more contexts you hear it in.
Collins, L., Trofmovich, P., White, J., Cardoso, W. and Horst, M. 2009: Some input on the easy/difficult grammar question: an empirical study. Modern Language Journal 93, 336-53.
Goldschneider, J. and DeKEyser, R. 2001: Explaining the `natural order of L2 morpheme acquisition' in English: a meta-analysis of multiple determinants. Language Learning 55 (Supplement 1), 26-77.
