This might be easier for English speakers to memorize for the rule on consonant mutation (where a word ending in an unvoiced consonant -- p, ç, t, k, f, h, s, ş -- undergoes a change to its suffix).
When a word ends in one of these letters and the suffix that means to be for third person singular (he/she/it) or third person plural (they) or the locative or ablative case endings* ...
... d changes to t as in:
he/she/it is -dir/-tir -dır/-tır -dür/tür -dur/-tur
they are -dirler/-tirler -dırlar/-tırlar -dürler-/türler -durlar/-turlar
EXAMPLE: Köpek küçük+tür. (The dog is small.)
-da/-ta (ending for locative case) (Vowel harmony applies as in examples for “to be” above.)
EXAMPLE: Deniz hangi taraf+ta? (Which side is the sea?)
-dan/-tan (ending for ablative case) (Vowel harmony applies as in examples for “to be” above.)
EXAMPLE: kiptap+tan (from the book)
For other suffix changes, just the following letters are affected:
ç to c [kazanç (profit) - kazancı (his profit)]
k to ğ [as in köpek (dog) - köpeğiniz (your dog)]
p to b [as in kitap (book) - kitabı (his book)]
t to d [as in kilit (lock) - kilidi (his lock)]
A great deal of the above comes from a very helpful website I refer to from time to time. It is called Manisa Turkish. Pages used for this comment have been pasted below:
http://www.turkishlanguage.co.uk/conmut.htm (Information on consonant mutation.)**
http://www.turkishlanguage.co.uk/tobepos.htm (Information on to be.)
Hope some of you find this helpful.
*If you need a review on either of those cases, click on the links below:
https://www.duolingo.com/skill/tr/Locative (for locative)
https://www.duolingo.com/skill/tr/Ablative) (for ablative)
**NOTE: For some reason Manisa Turkish does not include the letter "f" in its section on unvoiced and voiced consonants (which could be an inadvertent omission), but it is included at the very end of the page in the section on "Turkish Consonant Mutation Rules." It is also included in the tips and notes section for "Locative," so I made it part of the mnemonic at the top of this comment.
After writing the post above, I began wondering about the reasons for the consonant mutation rule and, in the process, I stumbled upon the following from (oddly enough) a website on English language and usage (https://english.stackexchange.com/). I found the information interesting, but more than that, often times when a student knows the reason behind a rule, it then becomes easier to remember and apply that rule to unfamiliar words in the future. So, I decided to post it below:
In English (and Latin, and most Indo-European languages, among many others), /b/ and /p/ are identical in pronunciation (both are bilabial stops), differing only in their Voice parameter; /b/ is Voiced, while /p/ is Voiceless.
It is a fact about the human vocal tract that consonant clusters that differ in Voice are difficult to pronounce, because changing from Voiced to Voiceless consonants requires independent movement of the larynx, which can be very hard to switch on and off at the millisecond timing required for consonant clusters.
Therefore there is a universal tendency in human language for consonant clusters to be either all Voiced or all Voiceless. However, what determines voicing when a cluster is formed from two dissimilarly-voiced consonants varies from language to language.
Note: The excerpt is from John Lawler, a retired English grammarian. If you would like to know more about his credentials/background, you can do so by clicking here: https://english.stackexchange.com/users/15299/john-lawler.