$\text{2.}$

$\text{→}$ The box is too heavy for Mike to lift.

$\text{→}$ The box isn't light enough for Mike to lift.

$\text{→}$ The box is so heavy that Mike can't lift.

$\text{→}$ It's such a heavy box that Mike can't lift.

$\text{3.}$

$\text{→}$ Tim is a smart baby but he is too young to walk.

$\text{→}$ Tim is a smat baby but he isn't old enough to walk.

$\text{→}$ Tim is a smart baby but he is so young that he can't walk.

2. The box is too heavy for mike to lift.

 The box is not light enough for Mike to lift.

 The box is so heavy that Mike can't lift.

 It's such a heavy box that Mike can't lift.

3. Tim is a smart baby but he is too young to walk.

 Tim is a smat baby but he is not old enough to walk.

 Tim is a smart baby but he is so young that he can't walk.