Good morning, Hardwork.
(1) Your sentences are very nice, but maybe they need to be
changed a little:
(a) Let's stand in the shade.
(i) I cannot explain why you need "the." Maybe someone else will.
(ii) Let's = Let us.
(b) I keep putting on and losing weight.
(i) When you get time, check your bilingual dictionary to study the
difference between "lose" and "loose"; "weight" and "wait."
Have a nice day!
P . S. Maybe this explains why you need "the."
(a) As you know, we use "the" when we refer to something that we know about: I bought A book yesterday. THE (that) book is very nice.
(b) You and a friend are walking on a very hot day. You and your friend see some shade under a tree. So you both know that there is some shade under the (that) tree. So you say:
Let's stand under the (that) shade (that we both see).
Student or Learner