My limit at the ask didn't fill. Did I do something wrong?

No; the price moved in that instant. Refresh, adjust a tick, resubmit. That's the system working.

Is a market order ever fine?

In a flagship ETF, mid-session, spread a few cents, modest size: practically yes. The limit habit exists because "practically" has exceptions you won't see coming.

What about fractional or savings-plan purchases where I can't set a type?

Automated plans execute at the broker's scheduled process (Chapter 16); you trade the control for automation, an accepted and usually fair deal at reputable brokers.

Should I use GTC to 'set and forget' buys?

For marketable limits, Day is right (you want it filled today). GTC belongs to resting-limit price opinions, used sparingly and reviewed.

Do these rules differ in the US vs Europe?

The mechanics are universal. Local color: US extended-hours sessions deserve extra avoidance, and European multi-venue routing makes the ISIN/listing check (step 1) slightly more important.

Placing Your First Order: Market, Limit and Stop

The order ticket, decoded

Every broker's ticket asks the same questions, whatever the layout:

Instrument: the exact fund and listing. Verify by ISIN and share class (Chapter 7): the most expensive "order error" is a perfect fill of the wrong product.
Side and size: buy/sell, in shares or (where supported) a cash amount.
Order type: market, limit, stop variants. The heart of this chapter.
Time in force: how long an unfilled order lives. Day (dies at the close) or GTC ("good till cancelled", typically parked up to 90 days). Defaults to Day at most brokers.
Venue: on multi-exchange European brokers, which order book (Chapters 12 and 14).

Three sell orders, one lurching market. The limit is the only one whose worst case is “nothing happened”. (A buyer’s market order in the same minute fills well — the gap punishes whichever side demands immediacy from a book that has emptied.)

Market orders: speed, unpriced

A market order guarantees execution, not price: it takes whatever the book offers, walking through price levels if the top one is thin. In a calm, tight, mainstream ETF at 15:45 CET, the result is nearly identical to a limit at the ask. So what's the problem?

The problem is that a market order's cost is conditional on the moment, and the bad moments are precisely the ones you don't foresee: a wide-spread minute, a quote gap around news, a fat-fingered size, an ETF whose underlying is asleep. Every notorious retail ETF mishap, including the 2010 Flash Crash fills at absurd prices (Chapter 81), was a market order meeting a bad minute. You will probably never hit one. The whole point of the limit habit is that "probably" costs nothing to remove.

Limit orders: the price-capped default

A buy limit at X fills only at X or lower; a sell limit at X only at X or higher. Two ways to use it:

The marketable limit (your default). Quote is 82.39 × 82.43; you want to buy now. Place the limit at 82.43 (or 82.45 for a touch of slack). Result: instant fill at the ask in normal conditions, and if the market lurches in exactly that second, your order simply waits or expires instead of chasing. You've paid nothing for insurance.
The resting limit (the patient variant). Placing a buy well below the market ("82.00, GTC") is not execution technique but a price opinion: you're hoping for a dip. Legitimate, occasionally satisfying, and statistically a drag for long-term buyers, since markets drift upward and unfilled orders become uninvested months (Chapter 17's lump-sum evidence applies). Use it knowingly, not as thrift theater.

One honest caveat: a limit order can go unfilled if the price moves away in the second you submit. The fix is undramatic: refresh the quote, adjust one tick, resubmit. That five-second chore is the entire "cost" of the professional default.

Stop orders: know them, then mostly decline

A stop (stop-loss) order rests dormant until the price touches your trigger, then becomes a market order. A stop-limit becomes a limit order instead. Traders use stops to cap losses on concentrated positions. For diversified ETF investors, the transfer fails on three counts:

A stop sells your plan, not your risk. The response this book builds to market falls is allocation and rebalancing (Chapters 19 and 60), not automated exits that convert declines into realized losses at the worst prices.
Stops become market orders in exactly the conditions where market orders misprice. In fast, gappy tape, your "protection at 78" can fill at 71. The Flash Crash turned thousands of well-meant stops into forced sales into a void.
Stop-limits fix the fill price and un-fix the protection: gap through your limit and you're still holding, now with an illusion punctured.

There are defensible niche uses (tactical positions in leveraged products someone shouldn't hold overnight anyway, Chapter 72). As a core-portfolio tool: the book's advice is a calm, informed no.

The order ticket, answered once. Highlighted fields are the settings that never change.

The complete first-order routine

Right product: ISIN + share class confirmed on the factsheet, listing chosen in your currency.
Right time: mid-session, underlying awake, never the first 15 minutes (Chapter 12).
Right type: limit at the current ask (buys) / bid (sells).
Time in force: Day.
Sanity-check the quote: spread looks normal for this fund? Size on the screen exceeds yours?
Submit, confirm the fill, done. Elapsed time after the third repetition: under a minute.

A worked example

Sofia buys €1,500 of a world ETF, quote 82.39 × 82.43, mid-afternoon.

She enters 18 shares (~€1,484), limit 82.45, Day. Filled in one second at 82.43. Cost of the "insurance" versus a market order: zero, since both would have filled at the ask.
Counterfactual bad minute: a headline hits as she clicks and the ask blinks to 84.10 for thirty seconds. Her order waits; the quote normalizes at 82.60; she nudges the limit and fills. The market-order version of her owns the same shares 2% more expensively and never knows why.

The habit's entire economics in one line: identical outcomes on the 99 normal days, and on the hundredth day the limit order is the difference between an anecdote and a shrug.