giving instruction and gerund

動名詞既有名詞的功用，因此，它當然可以做主詞，如：

Seeing is believing.（百聞不如一見）

不但如此，動名詞也可以做受詞，補語，如：

Complimenting is lying.(補語）
I hate telling stories.（受詞）

而pour the misture onto the tray and (put) it in the oven
這句是祈使句(imperative sentence)，祈使句動詞必須用原形動詞(bare infinitive)
祈使句常把主詞you省略，如：

Be quiet!
Never do it again.等等

但有時為了強調，也會將you寫在句首，如：
You come here!

說到這裏，相信你也明白為甚麼第二句，不用動名詞，而用原形動詞

希望幫到你！

We use imperatives for giving instructions. Imperatives = verb in the infinitive form, e.g. pour, mix, fry = tell people what to do;
You see, we don't use a gerund in this case, and thus there is no need to change the verb into gerund.

1. Spending time with animals = subject
was = verb great fun=complement

2. [You] pour the mixture
[You] is not written and is understood to be the subject of the action 'pour' the mixture and 'put' it in the oven.

But you can say 'Pouring the mixture onto the tray and putting it in the oven are two steps of the instructions.

i hope i have helped you.

2011-06-20 14:25:22 補充：
please do not delete answer. thanks!

Spending time with the animals was great fun.
"Spending time with the animals" --- Subject
其中Spending time with the animals是phrase，phrase可以現在分詞(present participle 或 過去分詞(past participle) 開頭。
Pour the mixture onto the tray.
這是命令句，動詞用原式，句頭其實隱藏了"You"字。

gerund 當然可以用作subject在一個句子裡,
但是它不可以用在giving instruction.
通常在instruction的句子中是無主語的. (如果要填主語, 主語通常是you.)

pour一字在該句中並不是主語.