An assumption particleたらis an abbreviation of たらば. ば is a conjunction particle express an assumption. たら was originally a conjugation of たり(an old auxiliary verb to express completion). So it is used as "When=at the time that it has finished" or "If=After the assumption would be realized ".
An assumption particleならis an abbreviation of ならば. なら was originally a conjugation of なり(an old auxiliary verb to express conclusion). So it is used as " If=in the case the fact would be changed ".
1.ならcannot be used in "found a new situation".
私<わたし>が窓<まど>を開<あ>け[たら]、目<め>の前<まえ>に紺碧<こんぺき>の海<うみ>が広<ひろ>がりました。(correct) When I opened the window, the dark blue sea spread out at hand.
私が窓を開ける[なら]、目の前に紺碧の海が広がりました。(incorrect)
2. ならcannot be used about "what you do not need to assume".
5時<じ>になっ[たら]帰<かえ>ります。I'll go back at 5(o'clock). (correct)
5時になる[なら]帰ります。(incorrect)
3. たらcannot be used in advise.
日本語を勉強したらitalkiがいいですよ。(incorrect)
日本語を勉強するならitalkiがいいですよ。(correct)If you want to learn Japanese (language) , italki is good.
4. The next phrase of たら is the thing after the former thing finished and the next phrase of なら is opposite.
飲ん[だら]乗るな。When you've drunk, don't drive (a car). (correct)
乗る[なら]飲むな。If you drive (a car), don't drink (alcohol). (correct)