Dentin is a major part of the tooth, located under the enamel.

He was walking along the coastline.

He called me back immediately after he hung up the phone.

He has already finished writing the report, so we use the present perfect form.

Right now, I am in Tokyo.

I am learning how to use the present participle in English.

The present tense is used to express things that are happening now or general facts.

My friend came from the Republic of Côte d'Ivoire.

He is always hanging out with bad friends. Truly, birds of a feather flock together.

She is learning the grammar of the present progressive form.