लोप

What drops

• The subject pronoun, when the verb makes it clear: कहाँ जा रहे हो? - जा रहा हूँ।
• The auxiliary after नहीं: मैं नहीं जाता।
• कि in reported speech: लगता है बारिश होगी।
• The whole verb in an answer: कितने बजे? - आठ।

What does not drop

तो in a conditional, the postposition on a noun, and the second half of a correlative pair. Dropping any of these leaves the sentence sounding broken rather than economical.

Recovering the ellipsis

Because the verb is final and carries person, gender and number, a listener can usually reconstruct the subject from it alone. This is why Hindi tolerates so much omission.