The Particle 了 le

It is not a tense marker (past), but an aspect or change marker.

• 了 (le) of change (at the end of the sentence): Indicates that a new situation appears.
• 我不去了。 (wǒ bù qù le.) (I won't go anymore / I've changed my mind).

• 我买了两本书。 (wǒ mǎi le liǎng běn shū.) (I bought two books).