Hi, I am Helen Zhang, a professional Chinese language teacher from eChineseonline.
1)  When he gets home, I will have already left.
      当他到家的时候，我已经离开了。
    Dān wǒ dào tā jiā de shíhou, wǒ yǐjīng líkāi le. 
      When you wake up, I will have already eaten.
      当你醒来的时候，我就吃完饭了。
    Dāng nǐ xǐng lái de shíhou, wǒ jiù chī wán fàn le,
In the two sentences above, "when"  is translated into “当…的时候(dāng,,, de shíhou)” in Chinese.

2) How do you say "in", as in: "in a certain period of time". 
    "In a certain period of time” can be translated into 在…之内 (zài … zhīnèi)
     I will go to Beijing in one week.
     一周之内我会去北京。
   Yì zhōu zhīnèi wǒ huì qù Běijīng.
    The store will open in two hours.
    这个商店两个小时之内会开门。
   Zhè gè shāngdiàn liǎng gè xiǎoshí zhīnèi huì kāimén.

3) How do you say "ago":
    "Ago” is translated into “…（以）前 (yǐqián)” 
   I meet my husband three years ago.
    三年前我遇到了我的丈夫。
   Sānnián qián wǒ yùdào le wǒ de zhàngfu.
    I started studying Chinese a month ago.
    一个月以前我开始学习汉语。
   Yí gè yuè yǐqián wǒ kāishǐ xuéxí Hànyǔ.

    Hope this is a help to you.