How well can a speaker of Hindi understand Marathi, Gujarati, Bengali, Nepali and Sinhalese?

Bengali is very different from Hindi and Hindi speakers can't understand it. Marathi though relatively closer to Hindi is still distinct enough to sound alien to Hindi speakers. Marathi speakers though can express themselves quite well in Hindi.

Gujarati is the closest to Hindi from all the above languages and a Hindi speaker ,with a little effort ,can more or less make out what is being said. More.

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