Learning any new language is difficult for a non-native speaker. Learning a new language is, in fact, one of the hardest things we ask adults to do. Fundamentally, a language is a tool that we use to communicate in order to accomplish tasks. With very few exceptions, all adults are competent speakers of their first, or native, languages, using them adeptly to get things done, from negotiating a business deal to ordering a cup of coffee. Nearly everything we do requires that we use language in some way, by listening, reading, writing, or speaking. But we don’t even notice that we’re using it.
However, when we go to learn another language, we notice immediately that we can’t actually do anything. A parallel example might make this clearer: Imagine you’ve driven a car with an automatic transmission most of your life, and then, one day, as an adult with dozens of years of driving experience under your belt, you find yourself needing to drive a car with a manual transmission. All of a sudden, driving, something at which you thought you were an expert, becomes absolutely impossible. You sit there, with the car bucking and the engine revving, breaking out in a cold sweat as it dawns on you that you can’t actually drive.
The same thing happens when a native speaker of any language goes to learn a language with which he or she has had no prior experience, and sadly for language learning, the learning curve is much steeper than getting the hang of driving a stick.
So, what makes learning a language especially hard?
When the language itself is very different from your own first language. The Foreign Service Institute categorizes languages in terms of difficulty based on how different they are from English and how many years of studying it takes native English speakers to acquire them. Languages that require learners to master a new writing system (e.g., Arabic), learn a new way of manipulating tone (e.g., Mandarin), manipulate pitch (e.g., Swedish), or master a grammatical concept absent from English (e.g., the notion of aspect in Russian) are more difficult for native English speakers than, say, learning Spanish, which is closer to English in all of those ways.
And what makes individual words hard?
Many things, including those described above, but especially how relevant those words are to a particular language learner’s context and how often he or she has heard/seen/used them. And, sure, words that have nonstandard spellings, colloquial meanings, multiple definitions, or include smaller words might be harder to master, all other things being equal. But the thing about learning a language is that things aren’t actually equal. The word “bench” in English, even though it can be used as a noun and a verb and has multiple definitions, is going to be easier for a non-native speaker to learn and use if it’s relevant to his or her needs. A courtroom clerk who asks people to approach the bench every day would never put that on a list of hard words.