Look at the letter b; at first it seems simple. There is only one sound for this one letter, the way a good letter should be. That is much easier to understand.
However the letter b also has an unusual -mb spelling-pronunciation pattern. The letter b is usually silent at the end of words in the combination -mb, in words of Old English origin. (The original is in brackets.) Examples of these words include: comb (camb), crumb (cruma ), dumb (dumb), lamb (lamb), numb (niman) and womb (wamb). The final letter b is pronounced in other words, even those from Old English, such as crab (crabba), rib (ribb), scab (sceabb) and sob (seofian). About 700 years ago lazy pronunciation dropped the /b/ sound, however the letter b was kept in the spelling.
Like those other Old English words, climb (climban) lost its /b/ sound. However the old pronunciation is more closely conserved in the related word clamber.
Many words with the -mb- combination in the middle of the word are of French or Old Norse origin. The /b/ sound is and has always been pronounced in these words.
There are some exceptions. The /b/ sound and letter b has been added to mumble (momelen). The words bomb (bombe), jamb (jambe) and tomb (tumbe), all from Norman French, also drop the /b/ sound in imitation of the Old English pattern.
The silent -b in thumb (thuma) and limb (lim) is strange since these words never had a /b/ sound. (This also applies to numb and crumb, above.) Adding a letter b to these words seems to have been a spelling mistake. A mistake that got repeated so often that people forgot there was ever another spelling.
The blends 'bl' and 'br' are common and often confused. Common words beginning with bl- are most often from Old English, with some Norman French and Old Norse.
Children learning the alphabet can confuse b and d. Draw a picture of a bed with the letters superimposed. This provides a way to visualise the problem. They do need to be taught that the -mb pattern at the end of a word has a silent letter b. Young Hong Kong students often confuse the blended sounds bl- and br- and these need to be practised. Compared with some other letters, b is not difficult.