They are used with subjunctive verbs when the clause indicates something generic or indefinite, etc. or if the clause describes quality or kind, etc. Otherwise, they are used with indicative verbs. Here are a few examples:
कल्पना कीजिए कि हम किसी गाव में हों जो बाहरी दुनिया से अलग हो – “Imagine that we are in some village that is isolated from the outside world”
In the previous example, a verb in the subjunctive mood (हो) was used in the relative clause “जो बाहरी दुनिया से अलग हो” (“that is isolated from the outside world”). It was used because the island is indefinite (no specific island is discussed) and the quality or type of island is discussed (i.e. a remote, isolated kind of island) and not its identity. This sentence could be rewritten with an adjective phrase instead of a relative clause.
Note that in Hindi, just as in English, the subjunctive mood is sometimes replaced with the indicative mood since the subjunctive mood is considered subtle and sophisticated. Thus, a speaker could say “जो बाहरी दुनिया से अलग है” in the previous example.
लेकिन इस शोध से ऐसी कोई बात सामने नहीं आई है जो आश्चर्य करने वाली हो – “However, nothing has come forward from this study which is surprising”
In the previous example, a verb in the subjunctive mood (हो) was used in the relative clause “जो आश्चर्य करने वाली हो“. The subjunctive mood is used because the sentence describes the kind of बात, namely, one that is not “आश्चर्य करने वाली” (surprising). Quality is emphasized, and not identity. It is a generic statement, not a specific statement. This sentence could be rewritten to use an adjective instead of a relative clause.
“स्मार्ट फ़ोन” ऐसा फ़ोन है जिसका प्रचालन तंत्र हो और जिसकी ऐसी सुविधाएं हों जो सामान्य फ़ोन की सुविधाओं से अधिक उन्नत हों – “A ‘smartphone’ is a phone that has an operating system and whose features are more advanced than the features of typical phones”
The previous sentence contains multiple relative clauses with verbs in the subjunctive mood. The first is “जिसका प्रचालन तंत्र हो” (“that has an operating system”). This clause contains the subjunctive verb हो because it is describing a kind of phone (i.e. phones with operating systems). This is reinforced by the adjective ऐसा, which means “such”, “this kind of”, etc. It is not describing a particular phone, but rather a class of phones. The next relative clause is “जिसकी ऐसी सुविधाएं हों” (“which has features …”). Again, this clause is describing quality, not identity. Finally, the clause “जो सामान्य फ़ोन की सुविधाओं से अधिक उन्नत हों” describes the kind of features that smartphones generally possess, and does not specify anything specific.
जो भी आप करना चाहते हो हम वही करेंगे – “We will do whatever you want to do”
In the previous example, the adverb भी emphasized the indefiniteness of the relative clause, i.e. whatever. Thus, the subjunctive mood was appropriate.
जो व्यक्ति हिंदी सीखना चाहता हो, उसको बहुत मेहनत करना होगा – “The person who wants to learn Hindi will have to do a lot of hard work”
In the previous example, no specific person was mentioned, but rather a general category of people (those who wish to learn Hindi).