Here is an overview of what you can do with TCG Letters searches.

plain words

find any letter that contains all of the words - in any order, in any field
format: words separated by spaces
example: lord of the rings find any letter that mentions these four words
example: tolkien edith find any letters to/from edith and tolkien, or mention both of them in the summary or transcript - if a letter was from Tolkien and mentioned Edith in the summary, this would match.

specific phrase

putting multiple words in quotes will match only that specific phrase in that word order
format: "phrase in quotes"
example: "farmer giles of ham" will not find a letter that just says "farmer giles" or "the farmer story about Giles"

letter by number

format: (any number)
example: 234 will find TCG letter 234, Carpenter's letter 234, and in addition will also find any letter that contains the text "234".

from someone

you can limit the results specifically to a specific sender. Note that there cannot be spaces between the from, the colon, and the name.
format: from:name
format: from:"full name"
example: from:tolkien find any letter where "tolkien" appears in the "from" field, so Christopher, JRR, Baillie all would match, for example
example: from:"Christopher Tolkien" find only letters where the "from" field contains exactly "Christopher Tolkien"

to someone

works just like "from" above
format: to:name
format: to:"full name"
example: to:tolkien find any letter where "tolkien" appears in the "to" field, so Christopher, JRR, Baillie all would match, for example
example: to:"Christopher Tolkien" only find letters where the "to" field contains exactly "Christopher Tolkien"

specific date

find letters that might be from a very specific date. Note that this will try to include letters that have a range that covers the date for letters that have uncertain "earliest date" and "latest date" entered in the database. The system is fairly good at converting most descriptive strings into actual dates - use quotes if you have spaces in the date description.
format: date:descriptive-string-with-no-spaces
format: date:"descriptive string with spaces"
example: date:1945 will match letters written during this year (or potentially written, for those with uncertain dates of composition)
example: date:1945-05-10 find only letters from May 10, 1945
example: date"February 1st 1914" find only letters from 1914-02-01

starting date

any letter from this date or forward (using same rules as "date" above)
format: start:descriptive-string-with-no-spaces
format: start:"descriptive string with spaces"
example: start:1951 find any letter from 1951-01-01 or later
example: start:"25 dec 1955" find any letter from Christmas 1955 onward

ending date

any letter from this date or prior (using same rules as "date" above).
format: end:descriptive-string-with-no-spaces
format: end:"descriptive string with spaces"
example: end:1951 find any letter from 1951-12-31 or earlier - note that if you just enter a year, it searches the entire year - starting from 12-31 and prior
example: end:"25 dec 1955" findany letter from Christmas 1955 or prior

Tag search

Letters have been tagged with various keywords to help find letters touching on certain topics
format: tag:descriptive-string-with-no-spaces
format: tag:"descriptive string with spaces"
example: tag:Publishing will find only letters with the Publishing tag - it will not find "publishing" mentioned anywhere else in the letter
example: tag:"The Lord of the Rings" will find letters with that multi-word tag exactly

Exclusions

You can look for letters that DO NOT match any of the above as well (eg "find letters that DO NOT match ...")
The general idea is that you put a minus sign - in front of things you want to exclude.
example: -edith will find letters that make no mention of "edith" in any field
example: -tag:gollum will find letters that do not have Gollum tag on them
example: -to:edith will find letters that were NOT sent to Edith
example: -from:"Christopher Tolkien" will find letters that were NOT written by Christopher
example: -date:"14 October 1945" will find letters that were not written on October 14, 1945 (note that this will also exclude uncertain letters whose "range of possible dates" includes this date)

Combining search filters

you can use any or all of the above, in any order, in your search
example: farmer giles from:unwin start:1945-01-01 end:"december 1 1950" (find letters containing "farmer" and "giles" that are from unwin (stanley or rayner) between Jan 1 1945 and December 1 1950)
example: Stanley Bliss (find letters from or to or mentioning "stanley" that also mention "bliss")

Other notes

• none of the search inputs are case sensitive. Feel free to capitalize if you like when typing in your search, but it doesn't matter.
• As mentioned above, be careful with white spaces.
• if you have any troubles at all, let me know exactly what you searched for and what you expected to see.

Here's an example of a complex query that you could try:
from:"christopher tolkien" father -lobdell start:1978 end:1978
Find letters written by Christopher Tolkien that mention his father, excluding anything mentioning/to/from Lobdell, from the year 1978