Vayakhel #1: The first word of parshat Vayakhel

is Vayakhel. Rashi writes a dikduk bit, explaining the word.

The pasuk (Shemot 35:1):

א וַיַּקְהֵל מֹשֶׁה, אֶת-כָּל-עֲדַת בְּנֵי יִשְׂרָאֵל--וַיֹּאמֶר אֲלֵהֶם: אֵלֶּה, הַדְּבָרִים, אֲשֶׁר-צִוָּה יְהוָה, לַעֲשֹׂת אֹתָם.

1 And Moses assembled all the congregation of the children of Israel, and said unto them: 'These are the words which the LORD hath commanded, that ye should do them.

and the Rashi, courtesy of Chabad:

1. Moses called… to assemble. Heb. וַיַּקְהֵל. [He assembled them] on the day after Yom Kippur, when he came down from the mountain. This [word] is a hiph’il [causative] expression [i.e., causing someone to do something], because one does not assemble people with [one’s] hands [i.e., directly], but they are assembled through one’s speech. Its Aramaic translation is וְאַכְנֵשׁ.

In general, the vowel pattern patach shva in the imperfect (usually future tense) denotes Hiph'il (a causative form). Thus, the patach under the yud of יַּקְהֵל and the shva under the quf of יַּקְהֵל it the imperfect Hiph'il. The vav hahipuch leading into וַיַּקְהֵל transforms this into the perfect (usually past tense) Hiph'il.

To reinforce his point, Rashi cites the Targum Onkelos who translated it וְאַכְנֵשׁ, which is the causative in Aramaic. The aleph patach in the beginning of the word does not denote a first person imperfect (I will gather) but is rather the `Aph'el form of Aramaic, one of the causative Aramaic forms. Perfect gather would be אַכְנֵשׁ, and if we for some reason we wanted imperfect, it would be initially *ye`achnesh - *יְאַכְנֵשׁ. (Asterisk means not actual form.) However, with the regular elision of aleph, it would become יַכְנֵשׁ

(The vav in וְאַכְנֵשׁ, by the way, is one which just means "and" - it is a vav hachibbur. It is not a vav hahipuch, and thus we can use the regular perfect form to connote perfect.)
If we were doing this with the Hebrew root קהל instead, we would get *יַקְהֵל <-- יְאַקְהֵל and with the addition of the vav hahipuch, it would be וַיַּקְהֵל. Thus, you can really recognize the imperfect Hiph'il because it looks exactly like imperfect Aramaic `Aph'el.