I’ve heard that को can be used to make a word definite, i.e. किताब = “a book”, किताब को = “the book”. Is that right? For instance, are both “मैं इसको कहता हूँ” and “मैं यह कहता हूँ” correct, and what is the difference?
Both sentences are grammatically correct, but the distinction is not their definiteness. Consider a few examples:
मैंने ऋचा से कहा कि … = “I told Richa that …”
वह मुझे सोनू कहती है = “She calls me ‘Sonu'”
उसने यह कहा कि – “She said that …”
Do you see how the postposition affects the meaning? कहना can mean “to say”, “to call”, “to tell”, etc. When it means “to tell”, the person who is told (the personal indirect object) is designated with से. When it means “to call” (i.e. “to name”), then the object is marked with को and the object complement is in the direct case (i.e. without any postposition). When it means “to say”, the direct object is in the direct case. In colloquial Hindi, some people use को instead of से, as in “मैंने उसको कहा कि …” – “I told him that …”.
Instead, definiteness is conveyed by qualifying nouns, such as “the red chair”, “that chair”, “your chair”, etc. Although Hindi lacks a definite article (“the” in English), it is perfectly capable of expressing definiteness in various ways.
When Hindi speakers do use को to make something definite, it usually occurs in situations like this:
मैंने आपको जो किताब दी थी, उसको पढ़िए – “Read the book that I gave you”, literally “The book that I gave you, read it”. The “it” is definite because it was mentioned previously in the context. It is referring back to the same book that was mentioned previously, so it is “definite” in that sense.
यह बहुत अच्छी किताब है. इसको पढ़ो. – “This is a very good book. Read it.” Again, the “it” refers to something mentioned previously.
It is not correct to teach people that को is some optional postposition that conveys definiteness if present, but indefiniteness if absent. Context always matters; my wife would say to me “पानी पियो“, regardless. I understand whether she means “drink some water” or “drink the water” by the context. She wouldn’t add को to make it definite; she would probably qualify it, e.g. “अपना पानी पियो“.
In Hindi, the object in an object-complement construction is marked with को, as in: “माँ की बहन को मौसी कहते हैं” – “They call one’s mother’s sister ‘mausi'”
Now, personal objects are usually marked with a postposition, and people are definite: “ऋचा को बुलाओ” – “Call Richa”, etc.
Adjectival conjunct verbs are very similar to object-complement constructions, so they often employ को, as in “मेज़ को साफ़ करो” – “Clean the table”.
Some verbs require their objects to be marked with a postposition. The postposition depends on the verb and the usage.
Here are some examples of verbs with postpositions:
मैंने उसको बताया
मैंने उससे कहा
मैं उस पर विश्वास करता हूं
मैंने उससे पूछा
मैंने उसको देखा (personal: “I saw him”; can be used impersonally, as in यह देखो, etc.)
So, to summarize, there are certain situations in which को is used (direct object, object-complement, with adjectival conjunct verbs, in relative clauses, backward references, etc.), and it might impart definiteness, but that is incidental. The usage of को is primarily grammatical. In other words, speakers don’t add or subtract it to express some subtle nuance; in most cases, they are required to do one or the other by the conventions of the language.