Swedish verbs are categorized into several groups and sub-groups. I would say that there is no one basic rule for present and past tense of verbs, and you have to know each of them by heart if you are to use them properly. One example, which is the easiest, is the first verb group. These are words that end with the letter a in its infinitive form. The word "show", meaning to "show someone or something" translates directly to the verb "visa". It then becomes Present tense: visar = showing Past tense: visade = showed Supine or Past Participle: har visat = has shown Other verb groups transform according to what letter they end, or how the last letter or vowel of the verb sounds. To answer your last question, irregular verbs are the most difficult to learn because they have separate rules, where the vowels change when the verb is used in present, past, and supine tenses.