In English, the letters mn only occur word-finally in a few cases. When they do, the n is silent. However, when the letters occur word-medially in a related form, they are both pronounced.
Here are a few examples:
The deletion of the word final nasal can be regarded as consonant cluster simplication or deletion. Without question, most languages do not allow the consonant cluster mn in word final position. Nevertheless, the spelling of words such as column and hymn suggests that the alveolar nasal was once pronounced.
The alveolar nasal is pronounced word-medially. Here the bilabial and alveolar nasals occur in different syllables. In autumnal, the bilabial nasal is in the coda of the first syllable and the alveolar nasal is in the onset of the second.
In Swedish the two nasals are pronounced word-finally. This is the case with namn (name) and famn (arms).
The spelling of words such as hymn and autumn reveals that the word-final nasal was once pronounced. Further evidence is provided by the related forms hymnal and autumnal in which the alveolar nasal is pronounced. It is possible to construct a rule for English which deletes the alveolar nasal in the cluster mn when it occurs word-finally. The rule can be written as follows: [mn] --> [m] / _#