Why is there a silent b in the word thumb.

There are quite a few English words that contain letters that are not suppose to be said including: debt and write. These left out letters can often tell us about the history and origin of the word and some places in the world has these which can tell us about who lived there at the time. This is called etymology.

Hope that this answer helped. ☺

Words over time changed from what they might have been. Ancient people could have said the word thumb with the b sound. But in current English we do not say it and this goes for other words as well. The word know has a silent k and w. It sounds like you are saying the word "no". There are other words that have silent letters but we do not know for sure why those silent letters are not said. 

See Explanations  below for answers

Explanation:

1. I felt/had felt nervous because I never was/had never

been on a plane before.

I felt nervous because I had never been on a plane before.

2. We read/had read a few travel books before we

decided/had decided to visit Patagonia.

We read a few travel books before we decided to visit Patagonia

3. She reserved/had reserved a place for her tent a few

weeks before, but when she arrived/had arrived, the

campsite was full.

She had reserved a place for her tent a few weeks before, but when she arrived, the campsite was full.

4 The area was/had been quite clean, as some

responsible residents cleaned/had cleaned all the

rubbish away.

The area was quite clean, as some responsible residents had cleaned all the rubbish away.

5 Did you ever find/Had you ever found the luggage

you lost/had lost in the airport?

Did you ever find the luggage you lost in the airport

6 I never thought/had never thought that travelling

could harm the environment so dramatically before I

read/had read that story in the paper.

I had never thought that travelling could harm the environment so dramatically before I read that story in the paper.

7 We planned/had planned a very detailed itinerary,

but then we didn’t have/hadn’t had the time to visit

all the places on our list.

We had planned a very detailed itinerary, but then we didn't have the time to visit all the places on our list.

8. I managed/had managed to communicate with the

local people because I learnt/had learnt a few words

before the trip.

I managed to communicate with the local people because I had learnt a few words before the trip.