In technical writing, where the process is more important than who is doing the activity, we might report that Three liters of fluid through porous glass beads. Regardless of the verb's purpose, only the auxiliary form of To be changes; the participle stays the same.
The To be will change form to indicate whether the subject is singular or plural: The To be verb can be combined with other modal forms (along with the past participle of the main verb) to convey other kinds of information.
To indicate the continuous aspect, add a form of the verb "to be" and a present participle to your main verb. (The above is colloquial, applying 2 past particple forms to "buy".) "Wrote" and "reread" sound equally important in the first sentence.
The perfect aspect is created with a form of the verb "to have" and a past participle. In the second, the past perfect form "had written" emphasizes the action "reread." Present Perfect refers to completed actions which endure to the present or whose effects are still relevant. (This could refer to any time in my past.) I have broken my leg.
For example: If you choose the present tense, as in Example 1.1, you're implying that the findings of the research are generally accepted, whereas the present perfect tense in 1.2 implies not only general acceptance but also current relevance and, possibly, the continuity of the findings as an authoritative statement on the causes of death.
On the other hand, the past tense in Example 1.3 emphasizes the finding at the time the research was conducted, rather than its current acceptance.Review carefully the material in our section on Subject-Verb Agreement, and notice how often the choices we make require a familiarity with these forms of the To be verb.We create simple yes/no questions by inverting the order of subject and the To be verb.The verb To be most frequently works in conjunction with another verb: He is playing the piano, She will be arriving this afternoon. Occasionally, though, the verb will stand by itself, alone, in a sentence.This is especially true in simple, brief answers to questions.The difference between the present tense and the present perfect (i.e.between consider and have considered ) is that the present perfect suggests that the current view has been held for some time. The future tense is standard in research proposals because they largely focus on plans for the future.The verb To be is said to be the most protean of the English language, constantly changing form, sometimes without much of a discernible pattern.Considering that we use it so often, it is really too bad that the verb To be has to be the most irregular, slippery verb in the language.The Greek sea god, Proteus, was (like the sea) capable of changing form in an instant.In order to get any decent information out of him, you had to grab him and hold on tight while he went through his various forms lion, wild boar, snake, tree, running stream it wasn't easy.