Hello

她保管(一定、肯定、定然)不知道 他们是爱人关系。 He certainly doesn’t know that they are a couple.
Tā bǎoguǎn bù zhīdao tāmen shì àiren guānxi.   （These three words are which what I often hear and use. For 定然, it's more formal and literary word, but seldom used in daily life.）

她爱打网球。 She loves playing tennis.
Tā aì dǎ wǎngqiú.

- 这个包多少钱？ How much does this bag cost?
- Zhè ge bǎo duōshǎo qián.
- 它值一快（一百）五十七元 It costs 157 yuan. 

(它值157元。is OK.  But if you are asking your friend about this. The question is correct. Thus, the answer you wrote here is too...."textbook style" XD. )

We tend to use like "这个包多少钱？--157元” simpler and you can easily add many your opinion in it. Like "157元，超便宜的！（Super cheap!）"、“157元，你觉得贵吗？(Do you think it is expensive?)”

这个汤太热（烫）的(了)。 This soup is too hot.  （热 is hot, 太热 is too hot but only used in weather! If saying about food is too hot to eat. Then we use 烫.）
Zhè ge tāng tài rè de

今天上午窝（我）头疼. This morning I had a headache.
Jīntiān shàngwǔ tóuténg.

作者写信了。 The author has written the letter.---Not really sure here. You mean, the author has already done the letter writing?-- If so, I suggest "作者已经把信写好了。"
Zuòzhě xiē xìn le.


那男孩了不得（超级、非常、极其）高兴。 The boy was extremely happy. （These three words are often used. And it's getting more formal--超级oral used, 非常normal used, 极其 formal)
Nà nánhái liǎobude gāoxìng

大家都到了吗？ Is everyone here?
Dàjiā dōu le ma?


我从来没坐过飞机。 I’ve never been on a plane.
Wǒ cónglái měi zuò guo fēijī.

Hope it helps!!

Ruby Lu