Verbs in which the first root letter is waw و are known as "initial waw verbs" and are subject to certain phonetic changes.
In the past tense they are conjugated normally, the irregularity occurs in the present tense. In this tense, the waw is dropped in the conjugation of form 1. The most common vowel sound found after the 2nd root in these cases is a kasra so the general pattern for an initial waw verb in the present tense becomes:

ya3ilu
compared to the common pattern of a regular verb in present tense form 1 - yaf3alu - the difference is mainly that having the 1st root letter (waw) dropped there are only two remaining, and also the kasra.

In other verb forms this does not occur, only with form 1. So an example in form two is the verb wajjaha (to send) which becomes yuwajjihu, the waw has been retained and it's conjugation is regular.

Some examples of verbs beginning with a waw:

وجد
found
وثق
trusted
واثق
agreed
وجّب
made an obligation
وجّه
sent
وقف
stood
وزن
weighed
وصف
described
وحّد
unified
وعد
promised
وضع
put
وصل
arrived

exercises:

conjugate the verbs in the sentences either keeping or dropping the waw according to what is appropriate, after considering the tense and the form:

1. She is arriving now
2. Are you putting the book on the table?
3. She promised me yesterday
4. He said :" i promise"
5. They are unifying the people
6. Yusuf described the house that he saw
7. Layla is describing the food
8. The merchant is weighing the cotton
9. All the girls are standing up
10. Everyone stood up.
11. He said: "I will not make it an obligation"
12. do you agree with me?
13. why do you trust that boy?
14. have you found my cat?