It seems that I have moved out of the “Other conjugations” section of dutchgrammar.com and into the irregular verbs section. Eek!
At first I was a bit concerned, since you are hit with a longgg list of irregular verbs which never seems to end. But then the following page showed the conjugation patterns, which helped to put things into like groups. So far I worked on two groups: e-ie-o and ie-oo-o.
The verb on the left is the infinitive (i.e. helpen = to help). The next column is for the singular past form, and the following columns (not shown) are for the plural past and the past participle. Those are the main things you need to memorize – single past, plural past, and past participle – since generally the present form is regular and there are only two forms in the simple past (singular and plural).
So basically, the past tense is the culprit. What dutchgrammar.com does is group like verbs together. So helpen, sterven, and werpen are in the same group: e – ie – o.
Infinitive: helpen, sterven, and werpen
Singular past: hielp, stierf*, and wierp
Plural past: hielpen, stierven, and wierpen
Past participle: geholpen, gestorven, and geworpen
* = in this form, v becomes f.
Next step – make flash cards for each group. But not tonight, so maybe this weekend.