NV Pronunciation
My conversational Spanish teacher from Costa Rica says that when an N falls in front of a V then the N is pronounced like an M. For example, the beginning of invierno (winter) would be pronounced im. What do you think? Have you ever heard of this?
Mi profesora de español conversacional de Costa Rica dice que cuando una N baja delante de un V entonces la N es pronunciada como un M. Por ejemplo, el principio del invierno (winter) sería pronunciado im. ¿Qué cree? ¿Ha oído hablar nunca de esto?
3 Answers
This is a well known (and reasonably well documented phenomenon). The term I would use (although there may be others) is "phonetic conditioning". Sounds are not (generally) uttered in isolation. Some sound precedes them and some (usually, other) sound follows them. A particular sound can be influenced by the preceding/following sound. Specifically, "n" when followed by a bilabial (such as "b/v", "p', or 'm') tends to be converted into 'm' ('n' and 'm' are both nasals and the most obvious distinction between them is that 'm' is bilabial while 'n' is not). However, there is a strong influence to convert the 'n' into a bilabial in anticipation of the following bilabial.
In the written language there are countless examples of this. Consider the prefix (inherited from the Latin) "in" meaning, loosely, "not" . In English we say "inhospitable" but we say "impossible". Both of these started out as "in-" plus something. In the former there is no "interference" form the "hos" syllable that follows but in the latter case the (bilabial) "p" that follows causes us to "blend" the sounds (converting the "n" to "m"). In most cases the spelling has been modified (evolved) to reflect this tendency (especially with prefixes such as "in-" and "un-" but, in other circumstances, the spelling does not reflect the pronunciation.
On a practical level, you could continue to use the sound of "n" (following the spelling and most people will not hear the difference (certainly not foreigners and very often not Spanish speakers) but, if you are interested in authenticity, it's pronounced as an "m" in those cases.
In the interest of completeness, what your teacher said was true but neglected to mention that "n" before any bilabial is pronounced as "m". Obviously before "b" since there is no distinction in Spanish between "b" and "v", but also before "p" and "m".
Hola, Helind, Perhaps in Costa Rica, they do pronounce the "n" before a "v" like an "m." So, maybe in your teacher's region it is perfectly acceptable. 
First time I've seen it. It may be easier, but I think it's not correct and it's not a rule.
