Latin/First Principal Part
I have two questions:
In Latin dictionaries you look for the "first person present active indicative" eg suscipio, if I understood it right. Why don't we look for the base form of the verb instead, eg suscipere?
So I looked up "suscipio" and it said "Third Conjugation IO-variation". What does "IO-variation" mean?
ANSWER: 1) "Suscipere" is not the base form of the verb. It is the present active infinitive. As you probably know, there are two principal stems (base forms, in your terminology), one for verb forms based upon the present stem (suscip-), and another for verb forms based upon the perfect stem (suscep-). You need to know the first person singular present active infinitive for the very reason that you ask about in your question #2.
2) If you consult your grammar, you will find that in the third conjugation of verbs, there are two variations, with two somewhat different sets of forms. There are those third-conjugation verbs that end in "-o" in the first person singular (such as "rego") and those that end in "-io" in the first person singular (such as "suscipio"). The rest of the verb forms differ somewhat according to whether the third-conjugation verb is an "-o" or an "-io" type.
To use your terminology the "base form" of the verb (e.g., "suscipere") will not tell you all that you need to know in order to conjugate all the forms of a given verb. You need also to know the first person singular present active indicative form (the first principal part listed in dictionaries) to give you all the information you need to conjugate the verb fully.
---------- FOLLOW-UP ----------
QUESTION: How will knowing the first person singular present active indicative form help me conjugate?
The first principal part (first person singular present indicative) will help you in two significant ways:
1) The first principal part will indicate whether the verb is active or deponent. For example: "Amo" is an active verb, having both active and passive forms. "Utor" is deponent, having passive forms only.
2) In the case of third-conjugation verbs, the first principal part will tell you which variation of conjugation to follow: "o" (regular) or "io" (I-stem). For example: "Rego" is is regular, conjugated somewhat differently from "facio", which is an I-stem.
Your grammar will help you with these principles. Charles Bennett's New Latin Grammar is a good one to start with: complete, but not overwhelming. It is available as a free eBook from http://www.gutenberg.org/ebooks/15665