I do speak both languages to a certain degree. My Nepali is definitely better in fluency, vocabulary, grammar application, but my Hindi grammar knowledge is more substantial. Furthermore I have studied the historical language development of Nepali and the Old Indian languages Sanskrit and Vedic (which is the oldest version of the Indian languages).
Both Nepali and Hindi obviously orginate from a common language ancestor in the past. To estimate when and where this language was originally spoken one has to do language comparison within the realm of Indian languages, as well as look at the available historical data.
Basically Nepali is the language that was spoken in Gurkha district, from where the Gurkha King conquered all of Nepal. But before that Nepali in its earlier development stages was a language „carried“ by the Hindu migrants who presumably started migrating into the Himal Pahads about 500 years ago after coming under pressure from the Muslim ruler/conquerors in the North-Indian flatlands. This migration of Hindu high-castes (Brahmin and Kshetri) who brought along with them their affiliated low caste servants (Dalits) happened slowly over centuries from West to East along the Himalayan mountain range.
Studying the different Pahadi dialects (of which Nepali would need to be regarded as one) – can teach you a lot about the history of the Hindu Migration through those hills. Of course before those Hindu castes arrived there were already other people living there, who either merged or fought with those new arrivals, but overall had to succumb to these new people who eventually claimed ownership over those hills.
And of course Hindi as well has its own history and has develped much further from that root language that Nepali and Hindi have both been derived from.
Still the connections are very visible and easily understandable. For a Nepali speaker who often grows up watching Indian Bollywood movies, Hindi is often the second language they are able to understand if not even speak fluently. On the other hand, if you grow up not being exposed to that language, you do only understand maybe 25% of what is being said in the other language. Indians who travel to Nepal for leisure or work, realise that once they are there – but also struggle to make the effort of learning Nepali, since their Hindi is understood everywhere.
Differences between pronunciation do exist, but they are minor and are really more like accents. People might find them noteworthy, but they do not impede communication. Vocabulary – I would estimate – overlaps for about 50%: Part of it are original words that have been sustained in its form and meaning in both languages and part of it are loaned words, which in most cases have been loaned from Hindi into Nepali.
Grammar is still recognizably similar, but different.
Let us compare a few phrases, so that you get an impression of the degree of difference here. I will chose phrases where the same (or very similar) kind of vocabulary is used, so that we can focus on grammar:
I want to learn Nepali.
Nepali: म नेपाली सिक्न चाहन्छु [ma nepaalii sikna caahanchu]
Hindi: मैं नेपाली सीखना चाहती हूं [mai~ nepaalii siikhnaa caahtii huu~] (this is the form for feminine first person singular)
म / मैं – I
नेपाली – Nepali
सिक्न / सीखना – to learn
चाहन्छु / चाहती हूं – (I) want
We can see that for each word the common origin is still very obvious – there have been mainly shifts for the vowels and something happened to the aspiration/non-aspiration of „k/kh“ in the word for „learning“.
The word order is also still the very same: Subject – Object – Verb (Infinitive + Auxiliary Verb)
सिक्न / सीखना are both in the infinitive, but it needs to be remarked that in Nepali there are two different Infinitive ending – in Hindi there is only one. The ending -न in Nepali is used in this kind of auxiliary verb construction, but the ending -नु is the one you will find in the dictionary.
चाहन्छु / चाहती हूं are both first person singular simple present. Obviously Hindi and Nepali both demand subjects for its verbs – meaning the pronoun म / मैं – I in these sentences is not optional. Furthermore both languages conjugate their verbs according to tense (which is also the case in English for example), and according to person (e.g. first person singular „I“ demands a different word-ending than the first person plural „We“). Hindi though is also very strict about conjugating according to Gender. It always matter if the subject is male or female. In Nepali this also exists, but it is only relavent for the 2nd and 3rd person singular – at least for the simple present.
If you look at the simple present construction closely you can see that Nepali creates the verb by combining the verb root चाह- with the conjugated form of हूनु [huunu] – „to be“ and adds a न् [n] in between. One needs to remark though that in Nepali (in contrast to Hindi there exist two versions of conjugated हूनु [huunu], which are used in different semantic contexts)
Hindi does construct the verb differently (but still recognizably very similarly). Instead of only using the word root चाह- it uses the present participle in combination with the conjugated form of होना [honaa] – „to be“.
What is your name?
Nepali: तिम्रो / तपार्इको नाम के हो ? [timro / tapaaiiko naam ke ho? ]
Hindi: तुमहारा / आपका नाम क्या है ? [ tumhaaraa / aapkaa naam kyaa hai?]
तिम्रो / तुमहारा – your (2nd person singular – 2nd level of formality)
तपार्इको / आपका – your (2nd person singular – 3rd level of formality)
नाम – name
के / क्या – what?
हो / है – (it) is
So again this is a good example to show how identical sometimes the grammar in Nepali and Hindi still is – the only differences in this sentence are derived from sound developments and loaning of words. The grammar has not shifted even a bit.
Both languages have different levels of formality for the 2nd person singular. Nepali has four, Hindi has three.
Both construct the genitive (the possessive case) by adding the postposition को / का – which you can see in the pronoun of the 3rd level of formality. And both have derived at an irregular construction of the possessive pronoun for the 2nd level of formality.
The kinship between the two question pronouns is obvious. As it is for the verb.
After giving you two such similar phrases I want to make you aware though, that this level of similarity is not always achieved. Here is a phrase to look at, that is very different in both languages.
I have returned after meeting with my friend.
म साथीसंगा भेटेर फर्केको छु [ma saathiisagaa bheTera pharkeko chu]
मैं दोस्त के साथ मिलकर लाैट गया [mai~ dost ke saath milkar lauT gayaa]
साथी / दोस्त – friend
-संगा / – के साथ – with
भेटेर / मिलकर – after meeting
फर्केको छु / लाैट गया – (I) have returned