Final nu sounds drop in SMG before fricatives and laterals. SMG students may find it simpler to learn this by thinking of addition of a nu before a stop or vowel.
Word-final -ν –n did a lot of work grammatically in Ancient Greek, serving as the marker for accusative singular in nouns whose stems ended in a vowel (e.g. λόγον lógon ‘word’, καρδίαν kardían ‘heart’, πρόσωπον prósōpon ‘face’), and figuring in various verb endings, such as 1sg middle past -μην –mēn or 1pl active present/past -μεν -men, and noun endings, e.g. genitive plural -ων –ōn. Nonetheless, it was phonetically weakened by the Hellenistic period before the consonants φ θ χ μ ν λ ρ σ ζ (ph th kh m n l r s z(d)) beginning the immediately following word, and ultimately was lost altogether, being retained before vowels and before the consonants π τ κ β δ γ (p t k b d g) and ψ ξ (ps ks, which are thus essentially the same environment as π κ p k). For the developments before π τ κ β δ γ (p t k b d g) and ψ ξ (ps ks), see the page on “Voicing of voiceless stop after nasal”.
The genitive plural in Standard Modern Greek (to the extent that it occurs at all — there are nouns that lack this form altogether, e.g. diminutives in -ακι –aki) has a final -ν –n, which is, however, not a direct retention from Ancient Greek but most likely represents a reintroduction of the earlier ending from the learnèd language, katharevousa; there are regional dialects with simply the expected -ω –o in the genitive plural (e.g. τω λόγω to loγo vs. Standard τω λόγων to loγon ‘of the words’).
Thus, final -ν -n is retained in definite article before stops and vowels
This combination was already affecting pronunciation in Ancient Greek, for there are inscriptions showing -μ –m for -ν –n before, e.g., π- p-, thus τομ πατἐρα tom patéra for τον πατέρα ton patéra.