Aspects are not so cut and dried
Aspects are not so cut and dried that one can say that one aspect is better or worse than another. I believe that an explanation for early-studies students of astrology has to always be couched very carefully. Please permit me to offer a couple of guidelines and then some comments.
HARD ASPECTS:
Conjunctions, oppositions, squares, semi-squares (45 degrees) and sesquiquadrates (135 degrees, also called square-and-a-half) are considered "hard" in that they are often linked to physical events and actions.
SOFT ASPECTS:
Trines and sextiles are often called "soft" because they involve communication, understanding, easy assimilation and accommodation.
OTHER ASPECTS:
The inconjunct (150 degrees) and semi-sextile (30 degrees) have been seen as fitting into a number of categories. Both are awkward and require lots of adjustment between the planets involved. Some astrologers believe they are always strong, others that they are always weak and secondary at best. There are also "harmonic" aspects built upon the division of the zodiac's 360 degrees by numbers like five, seven, nine, etc. wherein multiples of 40, 52, and 72 degrees are used. These can be quite useful if you know how to use them.
COMMENTS:
Now that we have briefly identified some common aspects, lets talk about the primary issue concerning aspects -- the planets that they link. The very nature of the planet can modify the nature of the aspect. Moon in a closing square to Sun can be quite positive, challenging one to live up to their public face, be responsible and work with their ego-will-focus in concert with their needs-emotions-habits. On the other hand an opposition between Saturn and Pluto can be brutal, whereas an opposition between Mercury and Moon might be conducive to mental-emotional swings in relationships such that life is changeable and flighty. An opening square by Venus to Mars can push one to take something desirable now and have to deal with consequences later, not always a harsh aspect but one that keeps one's life stirred up and full of tensions with others.
We have to also balance things like "orbs" (closeness or looseness of the aspect -- as this affects strength of effect), sign and house position, closeness to the angles, etc.
My advice is to 1) read alot, 2) initially adopt a set of guidelines that feel right for you, 3) do a lot of charts, and 4) modify and refine your guideline over time based on your experience and your continued reading and interaction with other astrologers. The study of astrology is something that evolves and deepens as you progress. You will both move to a mainstream position and understanding over time as well as develop unique interests and skills.
I wish you luck and excitement, bodhran, as you pursue your studies. Good luck. Dave