Words:
There are many words which have a similar origin, in some cases this leads you to wrong conclusions. Other words are just COMPLETELY different
zee - das Meer
meer - der See
winkelen - einkaufen
fietsen - Fahrrad fahren
Grammar:
- Dutch doesn't have cases, German does.
- Dutch and German both have three sexes (male, female, neuter) but in Dutch male and female or almost treated on the same footing (except for the use of pronouns). Unfortunately the sexes don't always agree, sometimes a word is male in one language and female in the other...
Phrasal construction:
- The word order in sentences and many phrases are pretty similar, except for the fact that Dutch people tend to write auxiliary verbs in relative clauses (and related structures) *before* the main verb, not after. Example:
Ich gehe in den Supermarkt, weil ich noch meinen Einkauf erledigen muß.
Ik ga naar de supermarkt, omdat ik nog mijn boodschappen moet doen.
- As a German the thing I have most problems with is the proper use of prepositions. It looks to me as if this is really entirely different...
- Some idioms are different (some very different, in some you only have minor differences)
als haringen in een ton vs. wie Ölsardinen
Conjugation:
- This is hard to compare. Some verbs are just entirely different. I have the impression that Dutch conjugation is simpler in many cases. If verbs are similar then usually also the perfect participle and the past forms are quite similar. But sometimes they are NOT and that makes mastering both languages simultaneously so difficult.
gaan - ging - gegaan vs. gehen - ging - gegangen
mogen - mocht - gemogen vs. mögen - mochte - gemocht
- What I found difficult personally is the different use of "to have"-forms and "to be"-forms in the perfect tense. A typical example would be
ik heb gewandeld vs. ich bin gewandert
I think these are the main similarities and differences. But I might well have forgotten to mention some of them...