Key concepts and its translation in Hindi Language:
1. Glass of water - पानी का गिलास (paani ka gilaas)
2. How to ask - कैसे पूछें (kaise poochen)

Usage examples and translate each example in Hindi language also:
1. Excuse me, can I have a glass of water, please? - क्षमा कीजिए, क्या मुझे एक पानी का गिलास मिल सकता है, कृपया? (Kshama kijiye, kya mujhe ek paani ka gilaas mil sakta hai, kripaya?)
2. May I request a glass of water, please? - क्या मैं एक पानी का गिलास मांग सकता हूँ, कृपया? (Kya main ek paani ka gilaas maang sakta hoon, kripaya?)
3. Could you bring me a glass of water, please? - क्या आप मुझे एक पानी का गिलास ला सकते हैं, कृपया? (Kya aap mujhe ek paani ka gilaas laa sakte hain, kripaya?)

Key mistakes:
1. Using incorrect word order: Instead of saying "Can I have a glass of water, please?", some learners might say "Can have I a glass of water, please?" It is important to maintain the correct word order.
2. Mispronunciation of words: Learners might mispronounce words like "glass" or "water." It is essential to practice the correct pronunciation to be easily understood.
3. Forgetting to use polite language: It is important to include polite phrases like "please" and "thank you" when asking for something in English.