When to 아/어/여 and when to use verb stems. Is there a trick?
So while learning many word modifiers and sentence patterns thanks to TTMIK. I noticed one of the biggest things I forget when recalling an ending is if I'm suppose to add the 아/어/여 and conjugate the verb or only use the stem of the verb. I was curious if there was a trick to this or not. I know that when conjugating to the past and present tenses you always add 아/어/여 but I'm asking specifically when you modify the verbs For example.

Tell someone to do something : Verb Stem + (으)세요.
(Correct) 오세요.
(Incorrect) [conjugating the verb stem] 와세요.

Ask someone to do something for you: 아/어/여 주세요.
(Correct) 해 주세요.
(Incorrect) [verb stem] 하 주세요.

I hope you understand what I'm trying to ask I know it can be confusing even just to understand a learners question. Is there a trick to this or better to just memorize? thank you!

Question

When to 아/어/여 and when to use verb stems. Is there a trick?
So while learning many word modifiers and sentence patterns thanks to TTMIK. I noticed one of the biggest things I forget when recalling an ending is if I'm suppose to add the 아/어/여 and conjugate the verb or only use the stem of the verb. I was curious if there was a trick to this or not. I know that when conjugating to the past and present tenses you always add 아/어/여 but I'm asking specifically when you modify the verbs For example.

Tell someone to do something :  Verb Stem + (으)세요.
(Correct)  오세요. 
(Incorrect) [conjugating the verb stem]  와세요.

Ask someone to do something for you: 아/어/여 주세요.
(Correct) 해 주세요.
(Incorrect) [verb stem] 하 주세요.

I hope you understand what I'm trying to ask I know it can be confusing even just to understand a learners question. Is there a trick to this or better to just memorize? thank you!

Answer

There are two cases of using 아/어/여 instead of verb stem.

First, 어/아/여 can be used to connecting verb and verb when the actions are sequential. 
I sat and watched TV. 
앉았다(past tense of 앚다) + 보았다(past tense of 보다) 
=> 앉(다)+아 보았다 => 앉아 보았다.
나는 TV를 앉아(서) 보았다.

I bought and read a book.
샀다(past tense of 사다) + 읽었다(past tense of 읽다) 
=> 사(다)+아 읽었다 => 사 읽었다.
나는 책을 사(서) 읽었다.

Second, -어/아/여 can be used with following auxiliary verbs
1. progress 진행
(어)가다 ex) 사 가다
(어)오다 ex) 사 오다

2. conclusion 종결
(어)내다 ex) 잡아 내다
(어)버리다 ex) 잡아 버리다

3. service 봉사
(어)주다 ex) 도와 주다
(어)드리다 ex) 도와 드리다

4. attempt 시행
(어)보다 ex) 먹어 보다

5. possession 보유
(어)두다 ex) 사 두다
(어)놓다 ex) 사 놓다
(어)가지다 ex) 사 가지다

6. passiveness 피동
(어)지다 ex) 작아 지다

7. emphasis 강세
(어)대다 ex) 먹어 대다

8. assumption 짐작
(어)보이다 ex) 작아 보이다

Answer

The trick is when you learn it to speak it. It helps me because when you speak it, it will feel strange on your tongue if it's incorrect. That's what I do if I'm not sure and it works 99% of the time.

Answer

I don't think there is any real trick or perfect rule. Start off by memorizing and eventually you'll have a good "feeling" for whether a new ending/expression is conjugated or just uses the stem. I learned primarily with TTMIK too, but supplemented it with http://www.howtostudykorean.com/ which presents grammar/vocabulary from the perspective of a native English speaker who become fluent in Korean.