Допомагати is an imperfective verb (aspect), and допомогти is a perfective verb (aspect). These are typical variations of verbs in Slavic languages, more in English here.
Imperfective verbs convey:
- actions and states in progress, just ongoing states and actions, with significant course (in opinion of the speaker);
- actions that serve as a background for other (perfective) actions, ex. коли зателефонували, я читав книгу — I was reading the book when the telephone rang;
- simultaneous actions, ex. читатиму книгу, коли брат писатиме листа — I will be reading the book while brother will be writing the letter;
- durative actions, lasting through some time, e.g. кричав — he was shouting, смикатиметься — it will be vibrating;
- aimless motions, ex. ходжу — I am walking here and there;
- multiple (iterative) actions, ex. дописувати — to insert many times to the text, виходитимемо — we will go out (many times);
- actions heading towards no or an unspecified purpose: писатиму листа — I will be writing the letter;
- continuous states, ex. стоятиму — I will be standing.
Perfective verbs can refer to the past or to the future, but not to present actions – an action happening now cannot be ended, so it cannot be perfective. Perfective verbs convey:
- states and actions that are seen as having finished (even if a second ago) or becoming so in a future time-point, have no significant course, have short duration or are treated as a whole by the speaker, ex. закричав — he shouted, перемішав — it will stir (only once);
- single-time actions, ex. дописати — to insert to the text, вийшов — he went out;
- actions whose goals are seen as having been achieved, even if with difficulty, ex. прочитав — I have read, дочитала — she finished reading and found what she had sought;
- circumstancial actions leading up to a state, ex. покохала — she came to love, зрозумієш — you (sg.) will understand, взнаємо — we will get to know;
- the beginning of the action or the state, ex. встану — I will stand up (and I will stand), почервонів — he reddened;
the end of the action or the state, ex. доспівай — sing until the end;
- actions executed in many places, on many objects or by many subjects at the same time, ex. повиносив — he carried out (many things), виламають — they will break out in many places, повирізати — to cut off many items;
- actions or states when they are seen as constituting a lexeme-specific block of time (Aktionsart), ex. постою — I will stand for a little time, побув — he was (there) for some time.