In some circumstances—but without the existing main clause "I
think"—all the sentences above could be used almost interchangeably.
Leave is one of the common verbs applying to travel
and scheduling (others, for example, are arrive, open,
close, begin, end, start, finish,
come, return) that permits the use of these four forms.
A television reporter might say, for example, sentence (a) or sentence
(b):
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(a)
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The president is leaving for the summit meeting
tomorrow,
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or
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(b)
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The president leaves for the summit meeting
tomorrow,
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to refer to the scheduled plans of the president.
Sentence (c) below predicts the event, but is slightly more formal
in style:
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(c)
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The president will leave for the summit meeting
tomorrow
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Sentence (d) below reports a prior arrangement or plan:
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(d)
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The president is going to leave for the summit
meeting tomorrow.
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In your sentences, the inclusion of "I think" colors the
dependent clause about the president somewhat. The event of his leaving
is not certain, but is based on the speaker抯 opinion. Sentences (e)
and (f) below are spoken at a certain time, say at 10:00 a.m. on Tuesday,
and refer to what the speaker believes has been scheduled for some time
the next day, Wednesday.
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(e)
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I think the president is leaving for the summit
meeting tomorrow.
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(f)
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I think the president leaves for the summit
meeting tomorrow.
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Sentence (g) below is a prediction using the opinion of the speaker
as a basis; there is no outside evidence for the statement. She is saying
that the president will make his decision to leave between now and tomorrow;
it抯 possible that the decision to leave has not yet been made. However,
she is predicting that the leave will indeed take place; perhaps she
thinks she knows a lot about human nature, or perhaps the tarot cards
say that the president will leave tomorrow. The speaker is asking us
to believe her conclusion, whatever her source of information, if any:
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(g)
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I think the president will leave for the summit
meeting tomorrow.
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Sentence (g) might also be spoken梬ith the spoken emphasis on the word
will梚n a situation to emphasize the speaker抯 belief,
contrary to others?beliefs, or in spite of difficulties. This use has
elements of the president抯 desire or willingness to leave (one factor
that evokes the use of will).
If the word will is emphasized in speech, as in (h):
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(h)
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I think the president will leave for the summit
meeting tomorrow.
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it could mean that the speaker believes that in spite of everything—in
spite of death threats from terrorists, his heart condition, the hurricane
at the meeting place—the president's departure tomorrow morning
is certain.
Sentence (i) below is also a prediction, but it is based more on some
outside evidence that the president has already planned to leave tomorrow,
or is scheduled to leave tomorrow. The speaker knows about some plans
which have been made, either because she is privy to some private plans
of the president, or she has been paying attention to news reports that
the listener doesn抰 know about:
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(i)
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I think the president is going to leave for
the summit meeting tomorrow.
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To explain these differences to students, a teacher might use a picture
of an Olympic diver standing on the edge of a diving board, poised
to
make his jump. Ask the students what they think. "I think he抯
going to jump" clearly shows that it is the intention
of the diver to jump. "I think he will jump"
is possible to show the speaker抯 prediction, but not the jumper抯
intention.
Neither "I think he jumps" nor "I think he is jumping"
can be used, as the action of jumping is not a scheduled event.
The verbs having to do with scheduling, such as leave
or other verbs that are clearly in a scheduling context (such as "She
graduates next spring") have, as you know, a wider range of tense
possibilities than others such as jump in the teaching
example above.