What are some specific features that make a particular language hard to learn? Please provide specific examples.

Polish is very hard to learn for most Romance language speakers (and I assume others) because it has so many consonant sounds - 38 by my count - and these sound very similar to untrained ears and can be stacked together in ways that would not be legal in most other languages. My wife is Polish and I have been trying to learn it for years and still can't distinguish between the three "sh"es and the three "ch"es. Conversely, English is hard for many Romance / Slavic speakers (and I assume others) because it has so many vowel sounds - 27 by my count. Compare this with the 5 or 6 sounds in many other languages and you can see why so many non-native speakers struggle with the difference between "balk" and "buck", for example.

There are no specific features. What is difficult for one person will be easy for another one. However, there are general features that might make life difficult. 1) The more symbols there are in the writing system, the more difficult the language is. This by default creates problems with hieroglyphics. This also goes for writing systems that require a more in-depth knowledge of written language and how it functions than just of its symbols (Chinese, Japanese, Hebrew). 2) Exceptions to rules. In this respect, Spanish is easier than Russian, French is easier than English. 3) Differences between written and pronounced text. For me, in this aspect, German, Spanish, Italian, Japanese, Chinese, Polish, Finno-Ugric languages, Kazakh language win. Russian, English, in some respect French lose. 4) Differences between language groups. If your native language is Indo-European, a Finno-Ugric morphology and syntax will be a pain for you, regardless of the fact that they use the familiar alphabet and many loans. 5) Intonation, tonal languages, sounds that are too-close-to-distinguish for a non-native speaker. 6) Naturally grammar categories add to the list of problems (grammar cases, verb tenses and the notion of time, gender category, differences between animate/inanimate), but in my experience those are not the worst, compared to the above. If you come from a language that is relatively simple in this aspect (English, Chinese), something like Russian or Estonian will be a pain. That is only from my experience and there are many people who've tackled more languages than I have so they certainly have a better picture, but since there are no answers to this question, I figure why don't I begin, at least.

The things which make a language difficult to learn are: 1) Grammar: If the grammar is similar to the persons mother tongue, then the learning process would be easier. For example, English and Spanish have similar grammar but English and Hindi do not. So an English speaker would find it easier to learn Spanish as compared to Hindi. 2) Vocabulary: If the vocabulary of the language is quite similar to the person's mother tongue than learning would be easy. But if the vocabulary is completely different, then learning would be difficult. For example: Bengali and Oriya have similarities in vocabulary, so its easy for an Oriya speaker to learn Bengali and vice versa. Same with English and some European languages. Also with Hindi and Marathi. But if an Hindi speaker wants to learn Chinese, or an English speaker Arabic, it would be difficult. 3) Alphabet: If the alphabet of the language is same as the mother tongue, then the process becomes simple. For example; English ,French, Spanish, Italian, etc use the same alphabet: Latin. Similarly, Hindi, Marathi, Sanskrit, Bhojpuri, Sindhi, etc also use the same alphabet Devanagari. But if a persons mother tongue is English and wants to learn Chinese, there would be lot of difficulty. Same with Hindi people to learn south Indian alphabets. Edit: Let's compare three languages, English, Hindi and Oriya sentences. English: He will go. (Male) She will go. (Female) Hindi: Woh jaegaa (male) Woh jaegi (female) Oriya: Se jiba. (Neutral) Another example: English: I will eat. (Neutral) Hindi: mai khaunga (male) Mai khaungi (female) Oriya: mu khaibi (neutral) We can see that Oriya sentences do not specify gender whereas Hindi sentences do. English sentences sometimes specify and sometimes don't.