Doen, zien, gaan, slaan, staan
What do these verbs have in common?
The similarity between gaan (to go), slaan (to hit) and staan (to stand) is obvious. Less obvious seems the presence of zien (to see) and doen (to do) in this group.
In fact, all these verbs have a regular present tense, which means they have no business being mentioned in the current topic about completely irregular verbs.
I chose to mention them here anyway, because their infinitives do not allow us to simply follow the usual steps for deriving the verb stem.
Stem = infinitive minus '-en'
The first thing you learn about the stem, is that it is the infinitive minus -en. It is not difficult to see that this is impossible for the verbs gaan, slaan, and staan.
The same goes for doen and zien. Although they both end in -en, the letter e is part of a vowel combination, not the usual mute e you find at the end of an infinitive.
Because oe in doen and ie in zien are vowel combinations, they are each considered as one long vowel. This is the reason why we cannot simply subtract -en.
Stem = infinitive minus '-n'
So what do we do instead? Very simple: We subtract -n. So the stem of zien is zie, and that of doen is doe.
Click here to see the conjugations of doen and zien.
But the stem of gaan is not gaa! And staa and slaa are also wrong. Recall, that a syllable never ends in a double vowel! The stems are thus ga, sla, and sta. In the 2nd and 3rd singular, however, we write double aa again: gaat, slaat, and staat. Because of the additional t, we need a double vowel to keep the sound long.
Click here to see the conjugations of gaan, slaan and staan.
Questions? 
