dave --
first, you write like a native speaker of english. so:
* if you are, in fact,
not a native speaker of english, then you deserve congratulations for learning to write so well.
* if you
are a native speaker of english, then
go ahead and use your ear for verb tenses (but don't dare use it for pronouns or modifiers!). in general, verb tenses are one of the very few areas of the language in which the spoken and written forms are largely the same.
davetzulin Wrote:when i read the mgmat verb tense mood guide it mentions that present perfect is used when the "action" or its "effect" lasts until now or even into the future.
well, yeah. verb tenses are very subtle and complicated -- so, when you write a book, you simply can't mention
everything about them. you have to strike some sort of balance between simplicity and accuracy; if we wrote sixty pages about the present perfect, then the end result would simply be that everybody would wind up confused.
also note that verb tenses are a minor topic on this test. they aren't tested very frequently in the first place -- and, when they are, they are usually tested along with areas that are more pervasive (parallelism, pronouns, modifiers, etc.) so, if they confuse you, the good news is that they don't matter as much as those topics do.
In your example of "evidence has suggested", you mention the evidence recently suggested these notions, but they don't suggest these notions anymore. shouldn't the notions (the effect) persist indefinitely? i never thought of present perfect as having a cut off time like past perfect.
so, here's the deal. these are definitely not rules that i've memorized ahead of time; i'm just thinking about a ton of different examples.
if you are using the present perfect for
point actions -- i.e., for things that happened
at a definite time in the past and don't persist (although their
effects may persist) -- then this is a non-issue in the first place, because those events occurred at one point and that's it. therefore, in the case of such events, the present perfect must refer to the impact of the event on the present situation.
for instance, if i say
i have earned a graduate degree in chemistry, then this is obviously something that happened in the past; the use of the present perfect indicates that it is somehow relevant to the current situation.
on the other hand, if you are talking about
states, conditions, titles, jobs, etc. -- i.e., things that persist over a period of time -- then there are two possible uses of the present perfect:
1/ (IF A SPECIFIC TIMEFRAME IS MENTIONED) continuing into the present. e.g.,
i have been self-employed for 18 years. (this means up until now; notice that "for 18 years" is a specific timeframe)
i have been lonely since she left (up until now; "since she left" is also a specific timeframe)
2/ (if no specific timeframe is mentioned) these things have been true in the past, but aren't anymore. e.g.,
i have been self-employed. (without the timeframe, this means i'm not self-employed anymore. if i were still self-employed, then i would either use a timeframe, like #1, or write "i
am self-employed")
i have been lonely. (i don't feel this way anymore. if i do, then i have to use a timeframe, as in #1, or use the present -- "i
am lonely.")
i will also note that writing a good chapter about verb tenses is extremely difficult, because ALL native speakers use verb tenses completely subconsciously. for instance, in any conversation between two native speakers of english, the above meanings will be instantly and accurately understood: any native speaker of english will understand, for instance, that "i've lived in florida" means i don't live there anymore, but "i've lived in florida for ten years" means i still live there. the frustrating thing is that 99.99% of those native speakers won't even begin to be able to explain
why these things mean what they mean -- so, to even attempt writing such a text demands and exceptional level of language awareness that very few people have (and that native speakers don't need anyway, unless they happen to make their living teaching english).
if i used simple past and said "embryologists find evidence" it changes the meaning.
i'm confused here -- are you saying that "find" is past tense? (it's not; past tense would be "found".)