The verb īnsum has the prefix in-. Prefixing in/in- to words, changes their meaning to ‘in’, ‘on’ et sim., or ‘un-’, ‘non’ et sim. (ɔ:¹ negation).² However, according to Wiktionary, the pronunciation of in changes in some context; this supported by Lewis.
At first, my understanding was that this pronunciation change – ɔ: from [ɪ] to [ĩː] – only occurs where the prefix is not the preposition in, but the actual prefix in-. Even though sounding the same, they have different roots: When meaning ‘in(side)’, it is merely the preposition prefixed; when meaning ‘not’/‘[negation]’, it has the Indo-European origin [*n̥-].5 As it turns out, the pronunciation change is merely a matter of morpheme interaction.
Non of my grammars, however, even the very detailed grammar by Nils Sjöstrand (Gleerups forlag, Malmö/Lund 1960) has any details on these two differences. The only thing I can find that is mildly relevant, is in Sjöstrand § 6.2, explaining the enlonging [I’m sorry, I am not sure what the correct linguistic term is] and nasalisation of vowels in front of ns and nf.
This means that pronunciation alone is not a clue as to whether or not one is dealing with in prefixed, or the prefix in-, as they both will have their pronunciations changed based on which morphemes follow. How can you know whether you are dealing with the preposition or the prefix, and thus the correct meaning of the word?
¹ This is not a smiley; it is the symbol meaning ‘that is’, ‘may be read as’ and similar.
² Egil Kraggerud & Bjørg Tosterud: Latinsk ordbok, Cappelen, Oslo 1998: ‘in i sammensetning’.
I have added the
negation tag, but am unsure as to its relevance. Advice on this is appreciated. It could very well be that the tag details should be updated to include questions such as this one.