Based on questions that got asked after class, I thought I would share some hints.
0) General tips for phonological problems are in ANGEL in the Assignments folder.
1) Somali - It's a neutralization problem like Russian (end of Tuesday handout). I will say that there ARE near minimal pairs for /m/ and /n/ in Somali, so your analysis should reflect that.
2) Nasal Place Assimilation - Many languages require that nasals and following stops be of the same place of articulation. In a language like Spanish, an /m/ can only come before stops /p,b/ (all labial) and /n/ can only come before /t,d/ (all alveolar).
Spanish allows /mb,mp,nt,nd/ but does not allow */mt,md,np,nb/. Similarly, Spanish allows /ñč,ñǰ/ phonetically but there are no /ñp/ or /ñt/ words. Before a stop, all Spanish nasals must be the same place of articulation as a following stop.
On the other hand, between vowels, Spanish allows all three nasals as in this minimal triple - cama /kama/ 'bed', cana /kana/ 'gray haired.f', caña /kaña/ 'cane' (bot). Therefore the nasals /m,n,ñ/ are phonemic in Spanish even though there are restrictions before stop consonants.
English also has nasal place assimilation as a rule which is seen in variants such as in- (indiscreet, intolerabale) vs. im- as in impossible, imbalance.
The word impossible can be analyzed as in+possible 'not possible'. The underlying form in- 'not' was chosen based on forms like in-alienable, in-edible where the prefix comes before a vowel.