Verb Tense

Present, past, or future action should be described consistently within the same sentence.

Incorrect: The geoscientist processes the data and stored the file on tape.

Correct: The geoscientist processes the data and stores the file on tape.

Simple past tense is correct for stating what was done, either by others or by you:

The solutions were heated to boiling.
Jones reviewed the literature and gathered much of this information.
We recently found that relativistic effects enhance the bond strength.
The structures were determined by neutron diffraction methods.

Present tense is correct for statements of fact:

Water freezes at 0°C.
Absolute rate constants for a wide variety of reactions are available.
Hyperbranched compounds are macromolecular compounds that contain a branching point in each structural unit.

Present and simple past tenses may both be correct for results, discussion, and conclusions:

The characteristics of the voltammetric wave indicate that electron transfer and breaking of the carbon-iodine bond are concerted.
IR spectroscopy shows that nitrates are adsorbed and are not removed by washing with distilled water.
Tests on core samples confirmed the presence of hydrocarbons.

Use the present perfect tense while presenting a completed study or its results.

The following data have been collected and processed [This implies that now the reader has the results in hands]