Difference in meaning between "δεν/δε" and "μην/μη" is your first question, the second is what is called "χρήση του τελικού "ν" "... Δε θα έρθω απόψε. / I won't show up tonight. Δεν ήρθες./ You didn't show (up). Μην έρθεις απόψε!/ Don't show up tonight! Μη 'ρθεις! / Don't show up. So, as you can see, they both show negation, a negative meaning, but "μην/μη" is used exclusively when you are using the imperative/προστακτική. "Μην έρθεις!" είναι προσταγή! "Δεν/δε'' is used the rest of the time. As to the τελικό νι, I am using the plainest examples here to get you started. (This is a confusing subject for a lot Greek native speakers, there are mnemonic rules to be mastered and it's not a given that everybody will follow the official usage changes in grammar as they take place.) We use the τελικό "ν", "για λόγους ευφωνίας", to make the language sound better (partly). That is why it is used when the next word starts with a vowel. Read it aloud and you can see the difference. Το λ ά θ ο ς "μη έρθεις" sounds too drawn out in Greek and lets all the air escape, too. It's "μην έρθεις!", "δεν ήρθες." So, the question is, when the next word starts with a consonant, do I use the "ν"? The answer is when it is one of the longer-sounding consonats (εξακολουθητικά). Which are those? "Β, γ, δ, θ, ζ, λ, π, μ, ν, ρ, σ, χ και φ"... (As opposed to the "στιγμιαία", "κ, π, τ, -μπ, -ντ-, -γκ, -τζ, -τσ" and the "διπλά'' , ξ (''κ+ σ'), ψ ("π+σ", that last less.) It is too tiring and closed to say το λ ά θ ο ς "μην 'ρθεις!" What a can of worms, huh? So, το "θ" είναι εξακολουθητικό, it's "Μη 'ρθεις", το "ρ" επίσης, λέμε "δε θα ΄ρθω"...