用it做形式主语时，用ing还是用ed？

首先，it不是take的逻辑主语。如果把条件状语改成从句，真正被省略的主语是we：
If we take all these factors into consideration, it is natural to draw such a conclusion that as far as his age is concerned, Tom is an excellent boy.
那么这种情况下应该用现在分词，而不是过去分词：
Taking all these factors into consideration, it is natural to draw such a conclusion that as far as his age is concerned, Tom is an excellent boy.
句子结构：
Taking all these factors into consideration 条件状语
it 形式主语，指代后置的主语不定式to draw such a conclusion
is 系动词
natural 表语
to draw such a conclusion 主语
that (as far as his age is concerned,) Tom is an excellent boy. 同位语从句说明conclusion
as far as his age is concerned 同位语从句中的状语从句

用过去分词短语作状语，分词短语的(被动语态)逻辑主语通常也是主句的主语。如：
Taken into consideration, all these factors lead us to draw such a conclusion that as far as his age is concerned, Tom is an excellent boy.
这句相当于：
If all these factors are taken into consideration, they lead us to draw such a conclusion that as far as his age is concerned, Tom is an excellent boy.

刚才桌子上有一张我最喜欢的电影明星海报。
A poster of my favorite movie star was on the table just a moment ago.
如果用你的表达方式：
A poster with my favorite movie star on it was on that table just now.
注意：
1. on后面一定要跟宾语。
2. just now表示的意思比”刚才”还要接近现在，是现在的一瞬间之前。除非这是你要表达的意思，还是用just a moment ago更好些。

第一个问题用taking。这里的it不是形式主语，形式主语类似于it is…for you to do。这里的it is…that是一种固定搭配。taking与it无关。take作为动词，放在句首都要变成动名词doing。。
第二个问题，首先with不对，应该用of，on lay也有问题，that改成the。最好用there be表示地点，简单，不易错。there was a poster of my favourite movie star on the table just now