Why can you say:
Or:
But you can’t say
Verb tense agreement.
I’ve being the contraction of ‘I have’ indicates current tense. Yesterday indicates a past tense time frame.
IMHO. IANA English teacher, just an avid reader.
It’s strange, though, because when learning foreign languages, we learned that the perfect tense is translated in three ways in English. I lost, I have lost, and I did lose. “I have lost” sounds like it happened just now, though.
But you can say
indicating a past tense time frame as well and be totally legit.
The important thing is the verb - known or lost. Why can you use a past tense agreemnt with one verb but not another?
On further cogitation…
‘I’ve lost my keys today’ is equally incorrect…
Guess we’ll just have to wait for that English teacher
It was a fair point and a good argument (although you cost me a beverage 'cause I took up your side) 
My girlfriend, who is not only English but an English teacher (as a foreign language) said your answer doesn’t work but is equally stumped as to why it’s wrong. One of her native Russian-speaking co-workers posed the question to her.
Close. I’ve never heard of “current tense,” but you’re on the right track.
Neither “I lost my keys” nor “I lost my keys yesterday” indicates the current status of they keys. The next sentence for either could be, “Fortunately, I found them this morning when I opened the front door.”
“I have lost my keys” is what grammarians call the “present perfect” which means that regardless of the fact that you are talking about something that happened in the past, your frame of reference is still the present time. In most constructions, “I have lost my keys” would strongly connote that the keys are still lost.
Adding a “yesterday” modifier conflicts with the “present time” frame of reference for the present perfect. You can say “I had lost my keys yesterday,” because that is the past perfect [del]tense[/del], and pretty much demands that a contrary current status is about to be described.
Edit: I slipped and called past perfect a “tense,” which should have been thoroughly beaten out of me by Ann Sharp, Ph.D., my Modern English Grammar and History of the English Language professor at Furman. Sorry, Dr. Sharp!
I forget what the lingo is, but I think it’s a matter of a single event vs. an ongoing event or condition, and past or present isn’t relevant.
I’ve lost my keys
I’ve got sixpence
Great answer, thanks, but why can’t I use a time frame such as ‘yesterday’ with the present perfect simple - such as ‘She has known about it for weeks’ - is this because ‘lost’ is a short action and so hasn’t been occuring for all this time, unlike the knowing which has, or is it some other reason?
Let’s start with this: the action is “lose.” “Lost” is its past tense and participle. I lose, you lose, he loses, I lost, you lost, he lost, I have lost, you have lost, he has lost.
Now, if I’m understanding your question, the answer is, yes, because the verb you have chosen to compare “lose” to describes (in that construction) an ongoing state of being rather than a single action.
Note that changing the construction to “My keys are lost” completely redefines the question, because now you are dealing with participles which, by and large, always describe ongoing states of being.
Actually, wouldn’t the perfect past (or whatever it’s called) be “knew” rather than “known”?
No. “Knew” is the simple past tense, “known” is the past participle, used for past perfect, present perfect, and future perfect constructions.
The past tense in English is used to indicate a single action that happened at some point in the past, while the present perfect indicates one or more actions that happened at some unspecified time in the past, and may have happened more than once. For example:
I ate at MacDonald’s.
I have eaten at MacDonald’s.
The former refers to a single instance of eating at MacDonald’s, while the latter refers to at least one instance of eating at MacDonald’s. “Have eaten” doesn’t refer to a specific time - it only means the speaker (or writer) isn’t a MacDonald’s virgin.
That’s why it makes sense to say “I lost my keys yesterday,” but not “I have lost my keys yesterday.” When a speaker says, “I lost,” he or she is referring to a specific action of losing keys, but when a speaker says, “I have lost,” it can refer to any number of times of losing the keys.
KneadToKnow’s explanation is correct in the specific instance, but I think it misconstrues the nature of the present perfect. It’s true that “I have lost my keys” in most contexts means that the speaker currently doesn’t have them. This is an inference, though. It isn’t in the meaning of the present perfect itself. Consider these two examples:
Q: Why aren’t you opening the door?
A : I have lost my keys.
vs.
Q: Have you ever lost anything important to you?
A : I have lost my keys.
In the former case, the answer means the speaker doesn’t have the keys, but in the latter it doesn’t. The words are exactly the same - the difference is context. In the former case, the only reason the speaker would give that particular answer would be to indicate that he or she doesn’t have the keys. No such inference can logically be made in the latter case. Note that in the former case there is little difference between the past and present perfect:
Q: Why aren’t you opening the door?
A: I lost my keys.
Here you go: Present Perfect Tense | ENGLISH PAGE
Buy Steven Pinker’s new book, The Stuff of Thought. It has a whole section on exactly why some constructions like this work and some don’t. Plus so much more!
Thanks, I think the web link got it.
After reading this thread, I just want to say that I’m so glad that I learned English as my native language and didn’t have to acquire it as an adult. My admiration for those who learn it as a second language is boundless; I’m not sure I could do it.
No way I could; Spanish damn near broke me and that was dead simple compared to English.
In my somewhat expert grammar opinion, one of the worst things you could do is automatically trust an English teacher on this subject. Think of them as Literature teachers, not Correct-English-Speaking teachers. I had one just walk away from me because she couldn’t admit she couldn’t answer my grammar question.