When conjugating a verb into the past simple, when do you use the -te or -de extension?

The best way to remember when to use -te(n) or -de(n), is with the mnemonic 't kofschip or 't fokschaap. Take the infinitive of the verb you wish to conjugate, and remove the -en at the end. What you have left is the technical root of the verb. Take the verbs: blaffen (to bark), krabben (to scratch) and leven (to live). The techinical roots would be: blaf- , krab-. and lev-. Note: The actual roots for verbs ending in v and z, we find by replacing the v and z with a f or s respectively: leef- for leven. However we are only concerned with the technical root here. To determine wheither we need -te or -de to form the past simple, we need to check if the final letter in the technical root is a consonant in the mnemonic 't kofschip. Yes? Then we need to add -te to form the past simple. No? Add -de. Take our examples: blaf (yes), krab (no), lev (no), so we get: blafte, krabde, and leefde. Note that we needed the roots and not the technical roots to add -te or -de to (leefde, not levde). There are exceptions to this rule, but in general it works quite well!