In order to study and remember the different conjugations of the Praesens Activum well, it is important to take a close look to the differences between the five conjugations.

THIRD CONJUGATION

Most verbs of the third conjugation don't have a basic vowel (see remark). The endings are -o, -s, -t, -mus, -tis and -nt (-re for the infinitive).
In all persons except for the first person singular and the infinitive, a vowel (i in second and third person singular and first and second person plural, u in third person plural, e in the infinitive) between the stem and the ending is needed to make the pronunciation possible. The example verb is regere (=to reign), the stem of that verb is reg.

Verbum regere - verb to reign (stem = reg)
rego - I reign
regis - you (s.) reign
regit - he/she/it reigns
regimus - we reign
regitis - you (pl.) reign
regunt - they reign

Some other verbs of the third conjugation:
apprehendere - apprehendo - to seize, to grasp
dicere - dico - to say, to speak
ducere - duco - to lead
mittere - mitto - to send
trahere - traho - to pull
vincere - vinco - to defeat

Remark: some verbs of the third conjugation do have a basic vowel, which is u. An auxiliary vowel (i) is added to enable the pronunciation, except for an e in the infinitive. The example verb is metuere (=to fear OR to doubt), the stem of that verb is met.

Verbum metuere - verb to fear OR to doubt (stem = met)
metuo - I fear
metuis - you (s.) fear
metuit - he/she/it fears
metuimus - we fear
metuitis - you (pl.) fear
metuunt - they fear

Some other verbs of the third conjugation:
minuere - minuo - to change OR to reduce OR to limit
statuere - statuo - to put, to place OR to decide OR to conclude