The postposition को has many uses in Hindi.
Definite Direct Object Marker
Certain verbs require postpositions to designate their direct object. The most common postposition used to mark a direct object is को. Other common postposition used to mark direct objects include का, से, and पर.
With certain direct objects, को is optional. For instance: थोड़ा पानी दो – “Give (me) some water”) and पानी को पियो – “Drink the water”; although this illustrates the difference, note that it is more common and appropriate to use demonstrative pronouns (यह/वह) to indicate definiteness: यह पानी पी लो – “Drink the/this water”.
The sentence without को treats the object पानी as a generic item – “Give (me) some water”. No particular water is being discussed.
The sentence with को treats the object पानी as a definite item – “Drink the water”. It is therefore analogous to the English definite article (“the”).
Therefore, को is appropriately used with objects that are inherently definite, such as people:
अंजलि को बुलाओ – “Call Anjali”.
With adjectival conjunct verbs, the को is optional: फ़र्श (को) साफ़ करो – “Clean the floor”.
With Ergative (ने) Constructions
को may optionally mark the object of a verb in an ergative construction. If this situation, the verb assumes the default form (third person masculine singular):
मैंने किताब को पढ़ा – “I read the book”
मैंने किताब पढ़ी – “I read the book”
Indirect Object Marker
Likewise, को also designates indirect objects.
मुझे थोड़ा पानी दे दो – “Give me some water”, e.g. “Give some water to me“.
Anaphoric Reference Marker
Since को designates definite objects, it is often used to make so-called “anaphoric” references (i.e., to refer to something that was mentioned earlier in the context):
मैंने तुम्हें जो पानी दिया उसको पी लो – “Drink the water which I gave to you”.
The sense is “The water which I gave to you, drink that water“. Here, को marks वह (“that” / “it”, oblique case उस) both as the direct object of पी लो, and as a anaphoric reference that refers backward to पानी.
Indirect Verb Constructions
को is used to form indirect verb constructions:
मुझे भूख लग रही है – “I’m hungry”
को is used with चाहिए to express want/need or obligation:
मुझे पानी चाहिए – “I want water”
मुझे पानी पीना चाहिए – “I should drink water”
With Temporal Adverbs
को may be used with temporal adverbs to indicate a time:
हम रात को पहुंच गए – “We arrived at night”
This is generally only used with specific times and times of day, such as with the words दोपहर, शनिवार.
को is generally not used to indicate destination, as “to” in English;
दिल्ली जाते हैं – “Let’s go to Delhi”
को is used to form several constructions with oblique infinitives:
तुम्हें देने को मेरे पास कुछ नहीं है – “I don’t have anything to give you”. (Explanatory Infinitive)
मैं जाने को तैयार नहीं हूँ – “I am not ready to go“. (Explanatory Infinitive)
उसको यहाँ आने को कहो – “Tell him to come here” (Complementary Infinitive)
Each of these uses of infinitives can also be used with के लिए.