How hard is it to learn a language? It depends on how close it is to your native tongue. But is it possible to describe a language’s difficulty in the abstract?
English-speakers often point to a language like Latin or Ancient Greek. Next to them, in one important respect, English is easy. The distinction involves a language’s “inflectional morphology”, or the bits and pieces added to a noun or adjective or verb to make it match up with other pieces in a sentence. An English verb has a maximum of five forms (speak, speaks, speaking, spoke, spoken), whereas verbs in Spanish or Latin can take dozens of forms. An English noun usually has only two forms (singular and plural), whereas the Greek or Russian noun takes numerous forms showing grammatical gender, number and case.
This kind of inflection is not a terrible proxy for that slippery idea of “difficulty”. Where are the world’s hardest languages, then? Is English one of them? One study, by Gary Lupyan and Rick Dale in 2010, looked closely at inflection. It found that highly inflected languages tend to be spoken by a small number of speakers, and have few neighbours. But languages with big groups of speakers, or many neighbouring languages, systematically tend to have fewer inflections. Why is that? The hypothesis is that as a language spreads over centuries, it is learned by many non-natives (trading partners, conquered subjects and the like). Adults, learning a foreign language imperfectly, avoid using non-necessary endings. And many endings in any language are non-necessary, if other clues (like word order in a sentence) can be recruited to do the same things that word endings do — say, distinguishing the subject of a sentence from its direct object. As languages spread and grow, they are more likely to rely on clues like word order than on word-endings. So “big” languages are “simple”. Under this schema, English fits both criteria: relatively big and relatively simple.
Of course, what’s hard about English is its irregularity, especially when it comes to spelling and pronunciation.