The different is in the (grammar situation) (إعراب). (لكن) is a conjunctive if the three condition are existed: 1- Followed with a word, not a sentence. 2- The sentence before it should be a (negative or inhibition). 3- ** NOT connected with (و). in that case, (و) will be a "conjunctive" and (لكن) is a (start and palinode letter) (حرف ابتداء واستدراك), and the next is an independent sentence. examples: ------------1------------- (لكن) is a conjunctive: لا تشرب اللبنَ لكن الماءَ Don't drink milk but water. ------------2------------- (لكن) is a (starte and palinode) letter: لا تشرب اللبنَ ولكن اشرب الماء Don't drink milk, but drink water. notice: - all of the above is about (ْلكن) without shadda on (ن) - in the meaning, i don't think that there is a apparent different in using لكن and ولكن. - sorry if my English isn't so good :\

ولكنْ: حرف ابتداء واستدراك. والواو للعطف. {مَا كَانَ مُحَمَّدٌ أَبَا أحَدٍ مِنْ رِجَالِكُمْ وَلَكِنْ رسُولَ اللَّهِ} لكنْ: حرف عطف. (بشرطين اضافيين: 1-ألا يكون المعطوف جملة ، 2-أن تسبق بنفي أو نهي). { لا تشرب اللبنَ لكن الماءَ } { ما جاءَ أحمد لكن خالد }