There are two basic sorts ("conjugations") of verbs in Japanese. Here we meet the first kind. These end in -u* and so let's call them U Verbs.
In the early stage of this course, we will only see verbs used in polite language (not in "plain", informal or intimate situations). These verbs usually have -mas- in their endings, for example - ます masu.
We need to know how to get from the form you find in the dictionary, ending in -u, to these -mas- endings. Fortunately, it is completely straightforward. Just change -u to -imas-:
asobu 'play (plain)' -> asobimasu 'play (polite)', nomu 'drink (plain)' -> nomimasu 'drink (polite), aru 'there is, have (plain)' -> arimasu 'there is, have (polite)', hanasu 'speak; release (plain)' -> hanashimasu 'speak; release (polite)', kau 'buy (plain)'-> kaimasu 'buy (polite)'
*but NOT -eru or -iru (see RU Verbs).
子供は遊びます。 Kodomo wa asobimasu. 'The children play.'
姉は話します。 Ane wa hanashimasu. 'My elder sister speaks.'
私は帰ります。 Watashi wa kaerimasu. 'I'm going home.'
僕は行きます。 Boku wa ikimasu. 'I'm going.'
全然分かりません。 Zenzen wakarimasen. 'I don't understand at all.'
子供 kodomo 'children'
僕 boku 'I, me' (used by male speakers)
全然 zenzen 'at all' (always used with a negative verb)
分かる wakaru 'understand'
Most verbs ending in -eru or -iru are RU Verbs, but there is a number which are U Verbs. Watch out for these! Two common ones are hairu 'enter' -> hairimasu and kaeru 'go home' -> kaerimasu.
There are also three irregular verbs. We've already met one: です desu. You need to look for that under である de aru in the dictionary (now you know where ではありません de wa arimasen comes from), but である de aru is only its plain form in written Japanese. In spoken Japanese だ da is used.
姉 ane 'elder sister'. Note that there is no word for 'sister' or 'brother'. You have to specify whether he/she is younger or older.