Actually, it wasn't there in Biblical Hebrew. I was a bit confused at first too.
It's pretty common to sea words written בתורה in a way known as חסר [khaser], meaning "lacking," as opposed to being written מלא [maleh], or "full." This was done with a purpose in mind (which is often lost in translation, sadly). Example:
Leviticus 20:26 וִהְיִיתֶם לִי קְדֹשִׁים, כִּי קָדוֹשׁ אֲנִי יְהוָה, "And ye shall be holy unto Me; for I the LORD am holy." You'll notice that, when referring to us the word "holy" is written חסר as קדֹש, but in reference to God it's written as קדוֹשׁ, which is מלא. This is to teach that, although we can achieve holiness, we can never have the same level of holiness as God himself.
Like I said, kinda lost in translation :)
Exactly. אתכם [etkhem] is "you" as the direct object, such asאני שולח אתכם [ani shole'akh etkhem], meaning "I am sending you." איתכם [itkhem] is "with you," such as אני שולח את הכלב איתכם [ani shole'akh et hakelev itkhem], meaning "I am sending the dog with you."