at first , normally 意思 and 意义 used in oral Maderin , 意味 used in writting Maderin . so mastering the usage of 意义 and 意思 is more useful for u ! 意义 & 意思 common : 1, noun.usually used in the question: 什么意思？ 什么意义？ 2, means "meaning" incommon :.usage : 1."What is the meaning ?"------这是什么意思？ when u don't understand the language or something ' s meaning , u must to use the word "意思" eg. ---你说的话是什么意思? ---我的意思是学汉语很难！ 2.“doing this thing is meaningful "----做这件事情是有意义的。 when u want to express that doing sth.is meaningful and u'd like to do it , u must use "意义"and normally "意义"is used in the phrase "有意义"（meaningful） eg.:---你为什么做这件事情？ ---因为做这件事情很有意义。（in here "有意义"becomes a adj. u can add "很"or "非常 “ in the front of the word.） ---你学汉语有什么意义呢？ ---我学好汉语可以到中国工作。 "意味"is a more difficult level word , seldomly use in oral Chinese . 1.noun. & verb. 2.normally used in the situation when have some commont on poems or literature works . or sth. can't express the meaning clearly. eg. 这首诗意味深长。(this poem expresses the full of meaning.) 我不明白他笑的意味。（I don't understand the implication of his smiling.）

the second one is the correct according to the Mandarin grammar system,But for the north people of China,for example of Beijing,they prefer to say "......去了" in oral language,so now people just accept this way.so choose what u like.but u should remember that when the thing was be done,u should add a "了"after the verb of the whole sentence.so the second one should be"我刚才去买东西了。"

these two phrase have about the same meaning,just remember that 1,"此"is used in writtern language,"这"is used in oral language .so almostly u will used"这" 2.the difference is :这（个)人-- 此人 ； 这(件)事-- 此事；这(件/个)东西---此物,and also,这人is short for 这个人，they are all correct!

In the acient time,an officer very treasured a young but poor talent man,each time the young man visited him,he always asked him to stay at his home for the night,and he prepared a "bed" for him, this kind of bed was put up when no used,put down when be used,so the officer put down the bed (下榻)when he asked the man to stay at his home.so after long time,people used "下"means having a good time with friends to go to a rasturant (下馆子)，this used normally in normal people, as the "下单"，you can cannect with the cooking thing,when u pour the food to the pot,it is said"下锅"，so “下”can be uesd wildly as sth.be done.nowadays,people use "下单"means the list is done!maybe what i said is nor very clear,but is hard to be clear on Chinese language,the amazing part is thar u can feel and understand it more or less.