Značka předmětu को

Not every object takes को

Hindi marks some direct objects with को and leaves others bare. The rule is roughly: people and definite things take it, indefinite things do not.

• मैंने किताब पढ़ी। - I read a book. (bare)
• मैंने लड़के को देखा। - I saw the boy. (person, so को)
• मैंने वह किताब पढ़ी। / मैंने उस किताब को पढ़ा। - both possible; the second insists on that particular book.

It changes the agreement

This matters in the ने construction. A bare object controls agreement; an object marked with को does not, and the verb falls back to the default masculine singular:

• मैंने चिट्ठी लिखी। - likhi, agreeing with chitthi.
• मैंने लड़की को देखा। - dekha, not dekhi, because ko blocks the agreement.

So को has consequences two words away, which is why it is worth a topic of its own.