Cash or accrual: the one decision the rest follows from. Under 26 CFR 1.446-1(c)(1)(i), the cash receipts and disbursements method includes items in gross income "for the taxable year in which actually or constructively received" and deducts expenditures "for the taxable year in which actually made." The phrase "constructively received" does real work: a check available to you on December 31 is income in that year whether or not you deposited it. Under 1.446-1(c)(1)(ii), an accrual method includes income "when all the events have occurred that fix the right to receive the income and the amount of the income can be determined with reasonable accuracy," and takes a liability into account when the events fixing it have occurred, the amount is determinable with reasonable accuracy, and economic performance has occurred.
Put plainly: cash follows the money, accrual follows the obligation. A business that invoices in December and is paid in February reports the income in the first year under accrual and the second under cash. Neither is more correct; they answer to different questions, and the difference is timing rather than total.
Why the choice matters beyond the tax return. Cash accounting is simpler and tracks the bank balance, which suits a business paid promptly. It also systematically misstates profitability where money and work are separated in time, because a good month of invoicing looks like nothing and the following month looks like a windfall. Accrual matches revenue to the period that earned it, which is what makes the numbers comparable month to month, at the cost of reporting profit on money not yet received. Certain taxpayers are restricted from the cash method entirely, and the restriction turns on a gross-receipts test that the IRS indexes annually, so an established business can grow into a requirement it did not have.
What "adequate records" actually means, which is a sufficiency standard rather than a list. Section 6001 requires every person liable for tax to "keep such records, render such statements, make such returns, and comply with such rules and regulations as the Secretary may from time to time prescribe." The regulation, 26 CFR 1.6001-1(a), gives the operative standard: keep "such permanent books of account or records, including inventories, as are sufficient to establish the amount of gross income, deductions, credits, or other matters required to be shown" on the return. Nothing prescribes a format, and 26 CFR 1.446-1(a)(2) says so explicitly: "no uniform method of accounting can be prescribed for all taxpayers. Each taxpayer shall adopt such forms and systems as are, in his judgment, best suited to his needs." One test constrains that freedom, in the next sentence: "no method of accounting is acceptable unless, in the opinion of the Commissioner, it clearly reflects income."
The practical translation of a sufficiency standard is that a spreadsheet is entirely acceptable and an incomplete one is not, and that the burden sits with the taxpayer. An expense that happened but cannot be established is not a deduction, which is why bookkeeping is best understood as the mechanism that creates deductions rather than as the paperwork that follows them.
How long to keep them, which is not the answer most people give. 26 CFR 1.6001-1(e) requires records to be available for inspection and "retained so long as the contents thereof may become material in the administration of any internal revenue law." That is a materiality standard with no fixed term. The commonly cited three years is the ordinary period for assessing additional tax, a different rule, and it does not describe records whose relevance outlives it: the cost basis of an asset stays material until the asset is sold, and records supporting a depreciation schedule stay material for the life of the schedule.
Changing method is a formal act. 26 CFR 1.446-1(e) requires a taxpayer to secure the Commissioner's consent before changing a method of accounting, and it is emphatic that "consent must be secured whether or not such method is proper or is permitted." A change from the cash method to an accrual method, or the reverse, is expressly a change of method. So is a change in the treatment of any material item, and the regulation defines a material item as one involving the proper time for including it in income or taking a deduction. The consequence for a small business is that switching how invoices are recorded is not a housekeeping decision, and doing it silently produces a return computed on a method the taxpayer is not on.
What a bookkeeping system needs to do, in practice. Separate the business's money from personal money, because a commingled account is the single largest obstacle to establishing anything later. Record every receipt and payment with its date, amount, counterparty and business purpose. Reconcile against the bank statement, which is what catches the omissions. Retain the substantiation rather than only the totals, since categories of expense with their own documentation rules, notably vehicle use and business meals, are defended by the record rather than by the number. And keep a reconciliation of any difference between the books and the return, which 26 CFR 1.446-1(a)(4) names as part of the accounting records themselves.