Remember I promised to explain the exceptions to the rule that the stem of a regular verb always remains the same? Well, occasionally, even a regular verb can be referred to as ‘radical-changing’ (radical-changing means that the stem is altered in some way – the word ‘radical’ comes from the Latin ‘radicalis’ meaning ‘by the roots’). This happens when either:
the spelling of the stem for one or more of the verb forms has to change in order to keep the pronunciation consistent, or
ii) the spelling remains the same, but the pronunciation of the stem changes – the stressed vowel changes from a closed quality to an open quality.
The first type of radical-changing verb occurs as a result of the peculiarities of hard and soft pronunciation of the letters ‘c’, and ‘g’. For example, the verb ‘agradecer’ – which means ‘to thank’ – requires a cedilla to be placed on the ‘c’ for the first person singular form – ‘agradeço’. This is done in order to keep the pronunciation of the stem unaltered (in this case, to keep the ‘c’ soft). Similarly, the verb ‘agir’ – meaning ‘to act’ – requires the ‘g’ to be changed to a ‘j’ for the first person singular (‘ajo’) – again, to keep the pronunciation of the stem consistent.
It is not really worth going into detail about the second type of radical-changing verb, because there are no rules to tell you when this change in pronunciation should happen. However, you may notice in reference works that certain forms of regular verbs whose stems remain orthographically consistent (that's a posh way of saying ‘are spelt the same’) are marked as ‘radical-changing’, in which case you will know that you need to alter the vowel quality of the stressed syllable from closed to open when you pronounce the affected forms of such verbs. Just to confuse matters further, some regular verbs are radical-changing in European Portuguese pronunciation, but not in Brazilian! Aaarrgh!
Please don't spend your nights lying awake worrying about radical-changing verbs! The above explanation is given for completeness, and for the benefit of those who want to understand the deeper implications of verb conjugation, but don't worry if you didn't fully understand it – I'm sure you could still survive a Portuguese conversation!