あなたがいなくなるととても寂しくなります。

you (referring to someone of equal or lower status); dear (what a wife calls a husband)

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

to be (of animate objects); to exist; to stay; (after the -te form of a verb) verb indicating continuing action or state (i.e. to be ..ing, to have been ..ing)

to become; to get; to grow; to be; to reach; to attain; to result in; to prove to be; to consist of; to be composed of; to succeed; to be complete; to change into; to be exchanged for; to play a role; to be promoted; to do ...

if; when; and; with; particle used for quoting (with speech, thoughts, etc.); quoting particle; promoted pawn; indicates question (sentence end)

very; awfully; exceedingly; (not) at all; by no means; simply (cannot)

lonely; lonesome; solitary; desolate