The first and second sentences would be correct if they were like this: お金がなくて何も買えない。 足が痛くて病院に行きたくても行けない。 This is just an observation (I'm not native but passed N1) so I can't say my explanation is absolute, but in certain contexts when you use ~くて + 動詞 you're implying that you can't do something even if you wanted to. You have no other choice. 何も買いません and 病院に行こうと思います implies that there IS a choice. You won't buy it (but you CAN buy it), You can go to a hospital (or back home, or to wherever) but you chose to go to the hospital. The presence of a choice opposes the meaning and gravity of the word expressed in ~くてwhich makes it ungrammatical because the meaning isn't logical. Another example: 本の内容が難しくて読みません(wrong) /// 本の内容が難しくて読めません(correct) 本の内容が難しいから読みません。(correct) The 2nd example means you want to read the book but you can't because it's too difficult. The 3rd means you won't read it because it's difficult. Hopefully someone comes along with a better explanation.