There are 3 groups of verbs in Japanese. The 2 main ones are Ichidan verbs (iru/eru verbs) and Godan verbs. The different groups conjugate in different ways, so it’s important to recognise the group that a verb belongs to.
These are pretty much all the other verbs, that don’t end in ‘iru’ or ‘eru’, but do always end in ‘u’. For example: のむ (nomu)、あるく (aruku)、きく (kiku)、つかう (tsukau)、しめる (shimeru) etc… All end in the vowel ‘u’. These are conjugated differently depending on the last syllable of the verb, whether it ends in tsu, ku, u, mu, ru, bu, su etc…
Godan verbs are also sometimes simply known as ‘Group 1’.
These verbs ALWAYS end in ‘iru’ or ‘eru’ (たべる and みる for example, meaning ‘to eat’ and ‘to see’). The ‘ru’ is generally completely dropped when conjugated, with the desired ended then simply added.
Ichidan verbs are also simply known as ‘Group 2’.
There are exceptions such as: はいる (hairu – to enter)、はしる (hashiru – to run) and かえる (kaeru – to return). Treat these as Godan verbs when conjugating.
Thankfully this final group consists of only 2 verbs – する and くる (suru and kuru). These conjugate in their own unique way but isn’t too complicated. There isn’t a huge list of irregulars to memorise, just suru, meaning ‘to do’ and kuru, meaning ‘to come’.
NOTE: Dictionary Form:
The verbs I have given as examples and the explanations I have given are in and regarding what’s known as the ‘dictionary form’ or ‘plain form’. It is so called the ‘dictionary form’ because it is the form given in Japanese dictionaries, and the ‘plain form’ because this is the most basic form of the verb. You will often see verbs in the ‘masu’ form (たべます、のみます、かきます) which is a more polite form. The plain form is for every day use talking with people you are familiar with.