My best explanation of imasu is used for living things. Like asoko ni hito ga imasu (There are people over there) Because you are talking about something living you use imasu. As in your example watashi wa tsukareteimasu you are talking about a person so you use imasu. So imasu = use for living objects is how I drilled that part of grammar into my head. I hope this helps some. :)

The way I think about it is that there is no equivalent of "to be" in Japanese. There are the words "iru" ("to exist" for living things) and "aru" ("to exist" for non-living things). Both can be also used as auxiliary verbs together with the -te form, where they have a different meaning (continous form and resultant state). In plain speech there's also the declarative particle "da", and in polite speech you conjugate verbs by adding -masu, and you add "desu" if there's no verb to conjugate. http://www.guidetojapanese.org/learn/grammar/stateofbeing http://www.guidetojapanese.org/learn/grammar/polite http://www.guidetojapanese.org/learn/grammar/teform So you can't say "kanashiimasu", because "kanashii" is an adjective, not a verb. You can't say "tsukareta desu", because "tsukareru" is a verb, and the past polite form is "tsukaremashita" ("I/you/whatever have become tired", -masu -> -mashita). "watashi wa tsukarete imasu" actually uses the polite form of the "-te iru" construction, which is for the continous form ("I am currently being tired"). So it doesn't really have anything to do with "to be". "desu" is just the polite marker in absence of a verb, so it hasn't got anything to de with "to be", either. I hope this makes some sense.

te-imasu is progressive form. expresses the fact that an action is presently in progress or is habitually repeated. verb1 hanasu=hanashi teimasu, v2 taberu=tabe teimau, v3=suru=shiteimasu. desu express a relationship between A and B. follow noun and adjective