The 't kofschip or 't fokschaap rule is a mnemonic

Mnemonic

A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...

 that determines the endings of a regular Dutch

Dutch language

Dutch is a West Germanic language and the native language of the majority of the population of the Netherlands, Belgium, and Suriname, the three member states of the Dutch Language Union. Most speakers live in the European Union, where it is a first language for about 23 million and a second...

 verb in the past simple indicative/subjunctive and the ending of the past participle. This rule should not be confused with the so-called T-rules

T-rules

The T-rules , are a set of conjugation rules used in the Dutch language to determine whether the second person singular/plural and the first and third person singular of a verb end in -t or not. These rules are related to the 't kofschip-rule, which is used to determine the verb end for past tenses...

 (t-regels).

Rule

The rule goes as follows:
For example:

Verb	Meaning	Verb root	Past simple	Past participle
rusten	to rest	rust-	ik rustte	gerust
werken	to work	werk-	ik werkte	gewerkt
spelen	to play	spel-	ik speelde	gespeeld
leven	to live	lev-	ik leefde	geleefd
crashen	to crash	crash-	ik crashte	gecrasht
bingoën	to play bingo	bingo-	ik bingode	gebingood
faxen	to fax	fax-	ik faxte	gefaxt

Silent letters at end of root

The rule is expressed in spelling of the verbs, but is actually related to pronunciation. So if the spelled root ends in a silent letter, this letter should be ignored in applying the rule. This includes also the apostrophe

Apostrophe

The apostrophe is a punctuation mark, and sometimes a diacritic mark, in languages that use the Latin alphabet or certain other alphabets...

 ‹'› occurring in some verb spellings.

For example:

Infinitive	Inf. pronunc.	Verb root	Past simple	Past simple pronunc.	Past participle	Past ptc. pronunc.
timen (to time)	/ˈtɑimə(n)/	time	ik timede	/ˈtɑimdə/	getimed	/ɣəˈtɑimt/
racen (to race)	/ˈreːsə(n)/	race	ik racete	/ˈreːstə/	geracet	/ɣəˈreːst/
deleten (to delete)	/diˈliːtə(n)/	delete	ik deletete	/diˈliːtə/	gedeletet	/ɣədiˈliːt/
sms'en (to send a text message)	/ɛsɛmˈɛsə(n)/	sms	ik sms'te	/ɛsɛmˈɛstə/	ge-sms't	/ɣə(ʔ)ɛsɛmˈɛst/
gsm'en (to phone using a mobile phone)	/ɣeːɛsˈɛmə(n)/	gsm	ik gsm'de	/ɣeːɛsˈɛmdə/	ge-gsm'd	/ɣəɣeːɛsˈɛmt/
petanquen (to play pétanque Pétanque Pétanque is a form of boules where the goal is, while standing inside a starting circle with both feet on the ground, to throw hollow metal balls as close as possible to a small wooden ball called a cochonnet or jack. It is also sometimes called a bouchon or le petit... )	/peːˈtɑŋkə(n)/	petanque	ik petanquete	/peːˈtɑŋktə/	gepetanquet	/ɣəpeːˈtɑŋkt/