Last night, I ran a group of five (three of whom were brand-new to the game) through a modified version of the Goblin Cave from the beginning of Lost Mines of Phandelver.
And from that entire sentence, the most important part, I think, is the word “modified.”
This particular adventure has a pretty firm three-session limit, and I knew I had exactly one session for the players to rescue Gundren from the cave. With 15 minutes left to go on the night, they still had a six-goblin encounter ahead of them. That would take entirely too long, so I changed it: One goblin was holding the barely-conscious Gundren over a 10-foot drop.
Much shorter, still an encounter, and still satisfying for the players – our brand-new sorcerer pulled a brilliant move, casting Sleep, Quickened Spell and then Misty Step to teleport over and grab the dwarf before he fell. And the players had no idea anything wasn’t “as it was supposed to be.”
Anyway, my point is don’t be afraid to tweak things behind the screen on the fly. Most likely your players will never know.
Now get out there and tell a story!