in this sentence “不着”can be translate to "cann't" "当累"->I think you want to say "when I am tired",当我累的时候 or you want to say"very tired",相当累

become happy:变得快乐

1. Some verbs plus 不着 means the negative forms. E.G. 一喝晚上就睡不着觉。 我找不着我的钱包了。 I can't find my wallet. 我点不着火。 I can't light a fire. 2. What do you mean by 当累? Maybe you can add some English explanation. The second sentence is correct. 3. 变得高兴 or 开心起来. If there is a context, we can give you more proper translation.