Try hard lest you ... fail. (Fill in the blank)

The correct word to fill in the blank is "had". The phrase "had better" is an idiomatic expression used to give strong advice or a recommendation, often implying a warning if the advice is not followed.

Here's a breakdown of its usage:

• Meaning: It means "should" or "ought to," but with a stronger sense of urgency or consequence.
• Structure: It is always followed by the bare infinitive (base form) of a verb, without "to" (e.g., had better call, had better go, had better study).
• Tense: Despite using "had," it refers to the present or future, not the past.
• Usage Context: It's commonly used in informal spoken English to suggest the best course of action.

In the given sentence, "You had better call a doctor," it means it is strongly advised that you call a doctor, implying that not doing so might lead to negative consequences.

Hasan, you and I borrow your pen?

The falcon hypersonic technology vehicle would be mankind’s dream aircraft in the days to come.

He will not can do this work.
= He will not cannot to do this work.

He will not can do this work.

= He can not do this work.

You may meet me tomorrow.  বাক্যের অর্থঃ তুমি আগামীকাল দেখা করতে পারো।