In the verb tener which is only form that not irregular?

The only form of the verb "tener" (to have) that is not irregular is the infinitive form:

tener

All other forms of the verb are irregular.

Here's a quick breakdown:

* Yo tengo (I have)

* Tú tienes (You have)

* Él/Ella/Usted tiene (He/She/You have)

* Nosotros tenemos (We have)

* Vosotros tenéis (You all have)

* Ellos/Ellas/Ustedes tienen (They/You all have)

As you can see, the stem "ten" changes in various forms, making them irregular.