I have some feelings (through the ways they answered my question) that some native speakers always think the following:
If we say "I was happy yesterday", that implies "I am not happy today"?
Maybe my feeling was wrong.
The USE 1: Simple past is used to show a completed action in the past & we know the time that the action completed.Source
I saw a movie yesterday.
The USE 2: But the Simple Past can also be used to describe past facts or generalizations which are no longer true. Source
So, when we say "He didn't like tomatoes before.", we imply that "Now he likes tomatoes"
Now, come back to the sentence "I was happy yesterday". It could mean:
The USE 1: that feeling completed in the past (Maybe, my girlfriend kissed me yesterday & therefore I felt happy at that time). But, today "I am still happy" (Maybe because I won a lottery, for example).
The USE 2: when you say "I was happy yesterday" you may contrast with the current situation. It could mean "I was happy yesterday but today I am not happy".
So, If I say "I was happy yesterday", does that always imply "I am not happy today"?
Also, What about "I was married 2 years ago."?
Let say 2 years ago I got a wedding with a lady & now that lady & I still live happily together. We can say "I am married" = "I have a wife now".
But can we say "I was married 2 years ago." but Now "I am still married". Does "I was married 2 years ago." imply "I divorced"?
Best Answer
There's implies, and then there's entails. When you say "always implies", I think you mean "entails".
Without further context, the sentence
implies but does not entail that your period of happiness has ended.
Implication and inference are soft, fuzzy things. That's the reason that we have those two words: I can imply something that you don't manage to infer. I can infer something that you don't mean to imply. There is no necessary connection between them.
Entailment is an essential, unavoidable thing. It's a consequence. If one thing entails another, that relationship is as strong and as certain as the relationship between cause and effect.
Context makes a difference. In the first series, today's misery stands in contrast to yesterday's happiness. We are certain that the period of happiness has ended. In the second, there is only one period of happiness that extends from the past, through the present, and possibly into the future.
I would not expect any native speaker to have a problem with either series of sentences above.