Writing Feedback / IELTS 1: A letter to the shop manager - 'it is broken' [11]

it's better to start writing such a formal letter with the expressions like; I am writing this letter (in relation to.../ regarding... / with regards to...)...

Although you want to make a complain, better not to use such expressions like "Now I am writing you to complain about it", because it sounds direct and impolite.

I think if you use some linking words between the sentences, that may sound more cohesive.
"It is my son's birthday five days later and I have promised him to bake his favorite cake for the party" is not RELEVANT.

Writing Feedback / IELTS;Some people think history has nothing or little to tell us [5]

There are some grammatical errors in a few sentences causing problems to convey the exact meaning, however, linking words are used well. Also, you could have written a few more sentences in conclusion paragraph.

"I give my view though following explanations".
"On the other hand, humans should not have take a step ahead without knowing the past."