I once had a Hungarian* friend who had come to live in England and, by the time I knew him, had learned to speak excellent English. He told me that English is one of the easiest languages to learn initially (IIRC he said “for the first year”), but one of the most difficult to fully master after that.
It is easy at first because the basic syntax of English is particularly simple. Nouns do not have to be declined at all (except for pluralization, which nearly always involves adding an “s” sound to the end, and the possessive which uses the “apostrophe s”**), there is virtually no grammatical gender (except for beings that are clearly differentiated by actual sex), there is really only one way of conjugating regular verbs (and even there, unlike many languages, the form of the verb does not change much, if at all, when used in a different “person” - I jump, you jump, he/she jumps, we jump, you jump, they jump), there are not particularly many irregular verbs, and most of them are not very irregular (e.g. although the past tense of “run” is “ran” rather than “runned”, in other respects it behaves pretty much like a regular verb). Most syntactic information in English is conveyed via a word’s position in a sentence. In other languages, you have to get the word order right and the inflection of many of the words. In English you scarcely have to worry about inflection, and even if you do get it wrong it is unlikely to cause any misunderstanding.
English is particularly difficult to fully master, however, because it is full of weird idioms: little phrases and constructions, often in very common use, that are completely irregular (syntactically, semantically, or both) and just have to be memorized one by one. (My Hungarian friend still struggled with certain idiomatic expressions, though he was getting there.) I am pretty sure I have read (and my Hungarian friend seemed to agree) that English has far more of these than most other languages.
And, of course, as others have pointed out, English spelling is full of weird irregularities, and is often not at all phonetic.
However, once someone has learned the relatively simple syntactical rules, and a good chunk of vocabulary of course (but that will be necessary for any language), they can make themselves understood in English, even if they don’t know many of the idioms. It will be obvious they are not native speakers, of course, but natives and even other speakers of “basic” English will be able to understand them perfectly well, and they will be able to understand most of what is said to them. English (spoken English at any rate), I am suggesting, is an easier language to “get by in” than many others.
Furthermore, I believe I have read that the syntactical rules of English are actually showing a trend toward becoming more simple over time in recent years. Some of those few irregular verbs are becoming regularized, and so forth (and already American spelling is not quite so irrational as British). This is thought to be, in large part, a result of the fact that it is spoken by so many as a second language. Non-native speakers are further simplifying the already simple syntax, making the language even easier to start to learn, and this is trickling back to the natives. (I wish I could remember a cite for this, but I can’t.)
Connected with this last point, I also recall reading somewhere that grammatical complexity of a language tends to be inversely proportional to the numbers of native speakers. The most syntactically complex languages known are usually those spoken by only a few hundred, or even only a few score, people deep in the jungles of Amazonia or New Guinea, or the central desserts of Australia.
- It is relevant that he was Hungarian, because although most European languages are more or less closely related to one another, Hungarian is not related to any of them. This means that, so far as similarity to his native lounge went, all European languages started off on a more or less equal footing for him. Nearly all European languages (together with many languages of western and southern Asia) are part of the Indo-European language family. To the best of my recollection, the only ones that are not Indo-European are Hungarian and Basque (and those are not related to one another). (I have a vague idea that maybe Finnish and Estonian might not be Indo-European, but I am by no means sure of that. In any case, they are still not related to Hungarian.)
**It is true that the rules for spelling possessive plurals, and possessives of words that end in s anyway can get confusing even for native English speakers, but this issue hardly impacts spoken English at all, and even in written English errors are usually unimportant and rarely affect comprehensibility.