Pronunciation is particular in English, as there are no set rules for pronouncing a specific letter. For example, vowels can differ given the slightest change of context. In the sentence, "A farmer's job is to produce produce," the "o" in the first "produce" is a short vowel while the second "o" is a long vowel. The student has to know common sentence structure and both noun and verb definitions of the word "produce" to accurately read the sentence.
There are particular consonant combinations unique to the English language. Friction words, for example, require different parts of the mouth to touch to make a specific sound. Most words with the letter "th" fall under this category. The "th" sound needs the tongue and teeth to touch. These sounds are unnatural in other languages and may be easily mispronounced.
English has numerous rules and exceptions for verb tense form. For example, the root word "sing" has different forms in past tense -- "sang" -- and past perfect -- "sung." Some words do not change at all. For example, the root word "read" is written the same way regardless if it's past or present tense but is pronounced differently.
English is in between many languages when denoting gender. Some languages do not distinguish between genders at all. For example, Mandarin Chinese only uses one term for "he" and "she" with the distinction being completely based on the context. Other languages, such as German, have three genders: masculine, feminine and neuter. Moreover, gender culture is also deeply infused in certain English slang terms. The statement, "You throw like a girl," would not make sense for foreigners who have different or no particular views on women.
When learning English in a particular region, the natural tendency is to pick up that region's accent. These accents have their own pronunciation styles and slang associated with them. For example, the New York accent is widely recognized in mainstream media. Even though the words and grammar stay relatively the same, there is a distinct style of pronunciation of certain vowels, such as "a" and "o." While it is not necessary to learn all the accents, it may be difficult to distinguish the same words from one accent to the other.