Many writers seem to assume that all verbs in a sentence must be in the same tense, but the tense for each verb should be appropriate to the context. Here are three examples of sentences in which a statement about a past event should reflect a continuous state. Discussion after each sentence explains the reasoning, and revisions illustrate it.
1. He discovered that in order to extract all the good compounds, you had to brew tea for twenty minutes at 80 degrees.
The discovery referenced in this sentence occurred in the past, but it describes something that was true before the discovery and during the discovery and will presumably remain true in perpetuity, so the verb pertaining to brewing should be in the present tense: “He discovered that in order to extract all the good compounds, you have to brew tea for twenty minutes at 80 degrees.”
2. For each area, they were then asked to indicate whether they believed their level of knowledge was adequate or requires improvement.
The querying of survey subjects described here happened in the past. If the respondents were asked this question again, their answer might be different, but at the time the question was asked, the survey sought a response about the current state of their thinking, so the verbs pertinent to that state should be in the present tense: “For each area, they were then asked to indicate whether they believe their level of knowledge is adequate or requires improvement.”
3. Jones asserted that these attributes of confidence were important because executives and directors can use them to assess and advance their organizations along the journey to realize their vision.
The attributes do not cease to be important just because Jones is not continuously asserting that fact. Barring an unlikely drastic change in the attitude of executives and directors toward them, they remain so, and the continuous state of this importance should be communicated by a verb in the present tense: “Jones asserted that these attributes of confidence are important because executives and directors can use them to assess and advance their organizations along the journey to realize their vision.”
Perfect!