Learn Spanish (Spain): Pongo el libro en la mesa.

Pongo comes from the verb poner (to put). It has an irregular I form (first-person singular). Instead of following the normal rule to make pono, it adds a -g-, making it pongo. The other present tense forms are regular, like pones (you put) and pone (he/she puts).

Doesn't en usually mean "in"? Why is it translated as "on" here?

The Spanish word en is very flexible and can mean "in", "on", or "at" depending on the context. Since a book is placed on top of a table rather than inside it, en la mesa translates naturally to "on the table".

Where is the word for "I" in this sentence?

In Spanish, you do not always need to say yo (I) because the verb ending already tells us who is doing the action. Since pongo can only mean "I put", adding yo is optional and usually dropped in everyday speech.

Can pongo also mean "I am putting"?

Yes! The simple present tense in Spanish can translate to both "I put" and "I am putting" in English. So Pongo el libro en la mesa can describe something you do as a habit, or an action you are doing right this second.