たまにはネイティブ向けの英英辞書を引いてみる
http://www.merriam-webster.com/dictionary/suppose
Oxford Advanced Learnersで引くと下記の通り。
子供にかんで含めるように平易に説明してくれるが、語彙を絞っている分、表現力が足りない感じは否めない。
1[intransitive, transitive]to think or believe that something is true or possible (based on the knowledge that you have)
2[transitive]to pretend that something is true; to imagine what would happen if something were truesuppose (that)…Suppose flights are fully booked on that day—which other day could we go?
3[intransitive, transitive]used to make a statement, request or suggestion less direct or less strong
m-wの英語辞書でsupposeを引くと下記の通り。
なにか仮説というかある考え、事実に依拠する、その前提につかまって考えるといったイメージ。ネイティブがよく使う、He is supposed to get to the station right now（彼はいま、駅に着いているはずなんだが。）.というニュアンスも生き生きと理解できる。会社の言葉にこだわるネイティブがいつも引いているm-wの辞書はお薦めです。
Definition of SUPPOSE
transitive verb
1
a : to lay down tentatively as a hypothesis, assumption, or proposal <suppose a fire broke out> <suppose you bring the salad>
b (1) : to hold as an opinion : believe <they supposed they were early> (2) : to think probable or in keeping with the facts <seems reasonable to suppose that he would profit>
3
intransitive verb
: conjecture, opine
